aconfig: flag java lib enable optimization

This change adds annotation in the generated code. If the flag is read
only, annotations @com.android.aconfig.annotations.AssumeTrueForR8 or
@com.android.aconfig.annotations.AssumeFalseForR8 will be added in to
the corresponding methods in the interface FeatureFlags, and class
Flags.

Test: atest aconfig aconfig.test.java
Bug: 295328116
Change-Id: Ic1b62a86f0945a5e72ae2c4987cce77fe3f172e9