Improve resulting APK size by using @Keep annotation

[LukasPaczos]

Closes #8978.

Up to this point, when Proguard was run we were keeping all class in the com.mapbox.mapboxsdk package, forbidding removal of unused methods or obfuscation. This PR proposes a switch to @Keep annotation to indicate which resources should be left untouched by Proguard. This targets any code that references Java classes, methods or fields from JNI context, or generally via reflection.

As this can be error-prone, because it's extremely easy to forget to add a @Keep annotation or an entry in the proguard-rules.pro file, which will result in runtime errors, this PR adds lint checks that will make the tests fail. Those checks search for:

• Any method that has a native modificator - those need to be kept at all cost
• Any field that has the type long and contains native substring in the name - by convention, this is how we name native pointer for peer objects. This should catch most of the cases, but is by no means sufficient, we need to pay attention to native pointers being kept ourselves.
• Any constructor that has a long type parameter which contains native substring in the name - similarly to above, by convention, these are constructor invoked from JNI.

For classes that are referenced by the JNI and are outside of the com.mapbox.mapboxsdk we should add an entry to the proguard-rules.pro file.

The most important thing is to remember about keeping Java fields, methods or constructors referenced from the native context, other than above, as those won't be highlighted by the lint check.

The quick tests show, that by minifying our test app we are able to:

• Decrease method references by 1179 (~15%)
• Remove 1118 unused methods (~20%)
• Decrease the size of the Mapbox Maps SDK Java classes in the dex file by 133.8KB (~23%)
• Get additional benefits of the obfuscation

What's worth noting is that this was done on the internal test app which uses the Maps SDK in nearly every possible way. Benefits in regular consumer apps will be even greater.

I tested the stability by running every single activity in the internal test app and exhausting its options, let me know if there is anything else I can do.

/cc @mapbox/maps-android @mapbox/gl-core

[tobrun]

I tested the stability by running every single activity in the internal test app and exhausting its options, let me know if there is anything else I can do.

Let's get this in the next alpha, this should buy us some time to track down issues that we haven't foreseen.

[tobrun]

Looks great!

[tobrun]

Do we really need to keep attributes as InnerClasses and Exceptions? can this be simplified?

[LukasPaczos]

I don't think we need, added them when testing things, good catch!