Introduce a java_system_features_srcs wrapper for codegen

Use soong to aggregate all system feature-related build flags as input
to the existing system feature codegen tool. Use this instead of the
more clumsy raw genrule variant. This is an effective no-op in terms of
generated code or framework impact, but should improve maintainability.

Test: m --no-skip-soong-tests
Bug: 203143243
Change-Id: I67158dd6f0454556f8d75685cfc86a68b3e47089