AAPT2: Produce Conditional Proguard Keep Rules

Add the option to produce keep rules that conditional keep based on
usage of R identifiers. This allows Proguard to potentially shrink more
code if resources are not used.

Currently only produces conditional rules for classes referenced in
layout resources because they are the most common and has the easiest
transitive usage chain to analyze.

Bug: 63628451
Test: make aapt2_tests and manual testing
Change-Id: I6c1af7affd64af40c80e004d8506a9463444b2c3