[Xamarin.Android.Build.Tasks] fix proguard rules for R8 compatibility

[jonathanpeppers]

Context: https://www.guardsquare.com/en/products/proguard/manual/usage#keepoptionmodifiers
Context: https://www.guardsquare.com/en/products/proguard/manual/examples
Fixes: #2684

Using R8 on a pre-Android 8.0 device was crashing with:

E/mono    (16220): Unhandled Exception:
E/mono    (16220): Java.Lang.LinkageError: no non-static method "Landroid/runtime/UncaughtExceptionHandler;.<init>()V"
E/mono    (16220):   at Java.Interop.JniEnvironment+InstanceMethods.GetMethodID (Java.Interop.JniObjectReference type, System.String name, System.String signature) [0x0005b] in <7d7bcc9ee9cc460db8abcdb9a9622733>:0
E/mono    (16220):   at Java.Interop.JniType.GetConstructor (System.String signature) [0x0000c] in <7d7bcc9ee9cc460db8abcdb9a9622733>:0
E/mono    (16220):   at Java.Interop.JniPeerMembers+JniInstanceMethods.GetConstructor (System.String signature) [0x00035] in <7d7bcc9ee9cc460db8abcdb9a9622733>:0
E/mono    (16220):   at Java.Interop.JniPeerMembers+JniInstanceMethods.FinishCreateInstance (System.String constructorSignature, Java.Interop.IJavaPeerable self, Java.Interop.JniArgumentValue* parameters) [0x00036] in <7d7bcc9ee9cc460db8abcdb9a9622733>:0
E/mono    (16220):   at Java.Lang.Object..ctor () [0x00054] in <d77389624c8c4948a12589c4bd4500eb>:0
E/mono    (16220):   at Android.Runtime.UncaughtExceptionHandler..ctor (Java.Lang.Thread+IUncaughtExceptionHandler defaultHandler) [0x00000] in <d77389624c8c4948a12589c4bd4500eb>:0
E/mono    (16220):   at Android.Runtime.JNIEnv.Initialize (Android.Runtime.JnienvInitializeArgs* args) [0x00202] in <d77389624c8c4948a12589c4bd4500eb>:0
E/mono    (16220):   --- End of managed Java.Lang.LinkageError stack trace ---
E/mono    (16220): java.lang.NoSuchMethodError: no non-static method "Landroid/runtime/UncaughtExceptionHandler;.<init>()V"
E/mono    (16220): 	at mono.android.Runtime.init(Native Method)
E/mono    (16220): 	at mono.MonoPackageManager.LoadApplication(:21)
E/mono    (16220): 	at mono.MonoRuntimeProvider.attachInfo(:1)
E/mono    (16220): 	at android.app.ActivityThread.installProvider(ActivityThread.java:5853)
E/mono    (16220): 	at android.app.ActivityThread.installContentProviders(ActivityThread.java:5445)
E/mono    (16220): 	at android.app.ActivityThread.handleBindApplication(ActivityThread.java:5384)
E/mono    (16220): 	at android.app.ActivityThread.-wrap2(ActivityThread.java)
E/mono    (16220): 	at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1545)
E/mono    (16220): 	at android.os.Handler.dispatchMessage(Handler.java:102)
E/mono    (16220): 	at android.os.Looper.loop(Looper.java:154)
E/mono    (16220): 	at android.app.ActivityThread.main(ActivityThread.java:6119)
E/mono    (16220): 	at java.lang.reflect.Method.invoke(Native Method)
E/mono    (16220): 	at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
E/mono    (16220): 	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)

Looking into it, the following method was present when using ProGuard,
but not R8:

#1              : (in Landroid/runtime/UncaughtExceptionHandler;)
  name          : '<init>'
  type          : '()V'
  access        : 0x10001 (PUBLIC CONSTRUCTOR)
  code          -
  registers     : 4
  ins           : 1
  outs          : 4
  insns size    : 22 16-bit code units
  catches       : (none)
  positions     :
    0x0000 line=22
    0x0003 line=23
    0x0010 line=24
  locals        :
    0x0000 - 0x0016 reg=3 this Landroid/runtime/UncaughtExceptionHandler;

Then I looked at proguard_xamarin.cfg, and noticed something strange:

-keep class android.runtime.** { <init>(***); }

It seemed this rule was being ignored? All the other working rules
were using <init>(...); to refer to the instance constructor.

When I reviewed ProGuard's documented syntax/grammar:

[@annotationtype] [[!]public|final|abstract|@ ...] [!]interface|class|enum classname
    [extends|implements [@annotationtype] classname]
[{
    [@annotationtype] [[!]public|private|protected|static|volatile|transient ...] <fields> |
                                                                    (fieldtype fieldname);
    [@annotationtype] [[!]public|private|protected|static|synchronized|native|abstract|strictfp ...] <methods> |
                                                                                        <init>(argumenttype,...) |
                                                                                        classname(argumenttype,...) |
                                                                                        (returntype methodname(argumenttype,...));
    [@annotationtype] [[!]public|private|protected|static ... ] *;
    ...
}]

It seems (...) is the proper way to specify a match against any set
of arguments. Looking through ProGuard's examples, I can't find any
usage of (***). It must happen to work, but is undocumented.

R8 does not appear to accept the (***) syntax at all.

Other changes

I expanded upon the DexUtils class so it can look for methods within
classes. I also adjusted one test to verify this problem is solved.

For comparison, attached dexdump output