Add Proguard rule for Crashlytics NDK
Prevents aggressive stripping of native methods in the JniNativeApi class.
# native libraries call FirebaseCrashlytics public methods dynamically via JNI.
-keep public class com.google.firebase.crashlytics.FirebaseCrashlytics { public *; }
-keep public class com.google.firebase.crashlytics.ndk.CrashpadMain { public *; }
+-keep public class com.google.firebase.crashlytics.ndk.JniNativeApi { native <methods>; }
