Clean up proguard.flags.

Splits the file between "proguard_common.flags" that other
variants should include, and "proguard.flags" that is used
for AOSP.

Remove reflection from FragmentService. This helps proguard, and
generally makes the code more pleasant to work with.

Bug: 257631899
Test: manually built and run
Change-Id: I014039a8187057bb85b526d3cdfb5ea1324a916b
Merged-In: I014039a8187057bb85b526d3cdfb5ea1324a916b