Keep Mapbox.java when obfuscating code

[tobrun]

This is needed to allow looking up this file from JNI code.
Refs https://github.com/mapbox/mapbox-gl-native/pull/15712/files

[alexshalamov]

verified locally, release and debug build test app works on pixel 2

[LukasPaczos]

Keeping just the getAssetManager method that is referenced from JNI is enough in this context and would allow obfuscating all other java methods.

[tobrun]

we also look up the class from JNI, so it needs to be kept as well

[LukasPaczos]

I believe that if you keep at least one method/field from the class, the class name is not going to be obfuscated. This allows all other methods that are not accessed from native to be obfuscated then.

[tobrun]

verified in https://github.com/mapbox/mapbox-gl-native/blob/201a53386f9f51cba2113b0a58541370d76c64af/platform/android/MapboxGLAndroidSDK/src/main/java/com/mapbox/mapboxsdk/Mapbox.java that this doesn't work.

[LukasPaczos]

Huh, interesting. It fails because if we're looking up a static method, JNI tries to verify the instance first with:
Caused by: java.lang.NoSuchMethodError: no static method "Lcom/mapbox/mapboxsdk/Mapbox;.hasInstance()Z"

The class is not obfuscated, but this internal, static hasInstance() method seems to be? Anyway 🍏

[tobrun]

we probably could fine tune this with using a specific proguard rule..