Filament references a few classes from native code and by reflections,
so when proguarding binaries we typically had to add an exception for
filament to make it run:
-keep class com.google.android.filament.** {*;}
In a compiled .dex file, the filament namespace takes about 120kb
(before compression), even if the classes aren't used.
To enable proguarding and stripping out unused filament classes,
introduce a UsedByNative and UsedByReflection annotation to explicitly
mark classes that need to be kept in the dex, so that the rest can be
potentially stripped out.
In my testing, this reduces the filament namespace in the .dex from
120kb->40kb, which translates to about 30kb apk size savings after
compression.
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