Add a simple metadata class option for system feature codegen

Add a `metadata_only` arg to the java_system_features_srcs rule
to optionally generate a very slimmed down output that maps
feature names to their new method names in the generated API surface.

This is useful for tooling like errorprone/lint to ensure any fixes
are correct, without pulling in the full codegen API surface.

Bug: 203143243
Test: m
Flag: build.RELEASE_USE_SYSTEM_FEATURE_BUILD_FLAGS
Change-Id: Ic9ec70d09cf2cfb11f9de939305dfa67ee4e28ae