Fix SDL File Handling

While updating the SDL2 recipe to version 2.0.4 in PR kivy#713, the
SDLActivity was not updated accordingly.

SDL 2.0.4 expects the Activity to have a method called
"openAPKExpansionInputStream" while this method was called
"openAPKExtensionInputStream" in older versions.

SDL2 is not able to read files when the method is missing.

pythonforandroid/bootstraps/sdl2/build/src/main/java/org/libsdl/app/SDLActivity.java

@@ -671,7 +671,7 @@ public static void pollInputDevices() {
 	public void keepActive() {
 	}
 
-    // APK extension files support
+    // APK expansion files support
 
     /** com.android.vending.expansion.zipfile.ZipResourceFile object or null. */
     private Object expansionFile;
@@ -680,16 +680,43 @@ public void keepActive() {
     private Method expansionFileMethod;
 
     /**
-     * This method is called by SDL using JNI.
+     * This method was called by SDL using JNI.
+     * @deprecated because of an incorrect name
      */
+    @Deprecated
     public InputStream openAPKExtensionInputStream(String fileName) throws IOException {
+        return openAPKExpansionInputStream(fileName);
+    }
+
+    /**
+     * This method is called by SDL using JNI.
+     * @return an InputStream on success or null if no expansion file was used.
+     * @throws IOException on errors. Message is set for the SDL error message.
+     */
+    public InputStream openAPKExpansionInputStream(String fileName) throws IOException {
         // Get a ZipResourceFile representing a merger of both the main and patch files
         if (expansionFile == null) {
-            Integer mainVersion = Integer.valueOf(nativeGetHint("SDL_ANDROID_APK_EXPANSION_MAIN_FILE_VERSION"));
-            Integer patchVersion = Integer.valueOf(nativeGetHint("SDL_ANDROID_APK_EXPANSION_PATCH_FILE_VERSION"));
+            String mainHint = nativeGetHint("SDL_ANDROID_APK_EXPANSION_MAIN_FILE_VERSION");
+            if (mainHint == null) {
+                return null; // no expansion use if no main version was set
+            }
+            String patchHint = nativeGetHint("SDL_ANDROID_APK_EXPANSION_PATCH_FILE_VERSION");
+            if (patchHint == null) {
+                return null; // no expansion use if no patch version was set
+            }
+
+            Integer mainVersion;
+            Integer patchVersion;
+            try {
+                mainVersion = Integer.valueOf(mainHint);
+                patchVersion = Integer.valueOf(patchHint);
+            } catch (NumberFormatException ex) {
+                ex.printStackTrace();
+                throw new IOException("No valid file versions set for APK expansion files", ex);
+            }
 
             try {
-                // To avoid direct dependency on Google APK extension library that is
+                // To avoid direct dependency on Google APK expansion library that is
                 // not a part of Android SDK we access it using reflection
                 expansionFile = Class.forName("com.android.vending.expansion.zipfile.APKExpansionSupport")
                     .getMethod("getAPKExpansionZipFile", Context.class, int.class, int.class)
@@ -701,6 +728,7 @@ public InputStream openAPKExtensionInputStream(String fileName) throws IOExcepti
                 ex.printStackTrace();
                 expansionFile = null;
                 expansionFileMethod = null;
+                throw new IOException("Could not access APK expansion support library", ex);
             }
         }
 
@@ -709,12 +737,14 @@ public InputStream openAPKExtensionInputStream(String fileName) throws IOExcepti
         try {
             fileStream = (InputStream)expansionFileMethod.invoke(expansionFile, fileName);
         } catch (Exception ex) {
+            // calling "getInputStream" failed
             ex.printStackTrace();
-            fileStream = null;
+            throw new IOException("Could not open stream from APK expansion file", ex);
         }
 
         if (fileStream == null) {
-            throw new IOException();
+            // calling "getInputStream" was successful but null was returned
+            throw new IOException("Could not find path in APK expansion file");
         }
 
         return fileStream;