Add system feature codegen for <unavailable-feature /> defs only

Add a new argument to SystemFeaturesGenerator that allows parsing *only*
the <unavailable-feature /> defs from provided feature XML files. This
is a safer incremental step, as any feature marked <unavailable /> by
any config file can never be overridden or updated (via mainline).

Test: atest systemfeatures-gen-golden-tests systemfeatures-gen-tests
Flag: build.RELEASE_USE_SYSTEM_FEATURE_XML_FOR_UNAVAILABLE_FEATURES
Bug: 203143243
Change-Id: I2c78e884c6f37c5adaacb0b19a4f7ba198449dac