Use Integer instead of int in map in flutter.groovy (flutter#141895)

packages Roller breakage 
https://ci.chromium.org/ui/p/flutter/builders/try/Linux_android%20android_build_all_packages%20master/5504/overview
Fixes flutter/issues/141897
```
FAILURE: Build failed with an exception.

* Where:
Script '/b/s/w/ir/x/w/flutter/packages/flutter_tools/gradle/src/main/groovy/flutter.groovy' line: 168

* What went wrong:
Could not compile script '/b/s/w/ir/x/w/flutter/packages/flutter_tools/gradle/src/main/groovy/flutter.groovy'.
> startup failed:
  script '/b/s/w/ir/x/w/flutter/packages/flutter_tools/gradle/src/main/groovy/flutter.groovy': 168: primitive type parameters not allowed here;
     solution: use the corresponding wrapper type, such as Integer for int @ line 168, column 41.
     e static final Map<String, int> ABI_VERS
```

Covered by tests in packages.

packages/flutter_tools/gradle/src/main/groovy/flutter.groovy

@@ -165,7 +165,7 @@ class FlutterPlugin implements Plugin<Project> {
      * For each APK variant, use the following versions to override the version of the Universal APK.
      * Otherwise, the Play Store will complain that the APK variants have the same version.
      */
-    private static final Map<String, int> ABI_VERSION = [
+    private static final Map<String, Integer> ABI_VERSION = [
         (ARCH_ARM32)        : 1,
         (ARCH_ARM64)        : 2,
         (ARCH_X86)          : 3,