Add a method to get all compile-time available system features

This method is really only useful for SystemConfig, where we currently
parse available/unavailable features from the image. The method
lets us seed the SystemConfig available feature set with those defined
and compiled into the framework.

Flag: EXEMPT tool update
Test: atest systemfeatures-gen-tests systemfeatures-gen-golden-tests
Bug: 203143243
Change-Id: I627464b5a9d50137270d5658f7c4ca6b1d1f8cfe