Pipe system feature build flags into codegen

Route build flag values for currently defined features into the
systemfeature codegen tool. Their impact on codegen is still
conditioned on the RELEASE_USE_SYSTEM_FEATURE_BUILD_FLAGS flag.

A follow-up change will simplify this by aggregating all such flags
in a native soong module that feeds directly into the codegen tool.
For now, we have a relatively small list of build-time defined
features, but additions will remain tedious until we do automatic
aggregation.

Flag: build.RELEASE_USE_SYSTEM_FEATURE_BUILD_FLAGS
Bug: 203143243
Test: m framework
Change-Id: Iffc607fdcbe99216829b4b01b6c922d8d66ef1ea