Add feature XML file parsing support

Add the ability to provide XML feature definition files (those used by
SystemConfig/PackageManager and distributed on devices) as inputs to
SystemFeaturesGenerator via the `--feature-xml-files` arg.

Note: This arg is not yet integrated into production codegen, but will
be used in future changes to scrape `<unavailable-feature />` defs in
product as inputs to codegen, behind a trunk stable build flag.

Bug: 203143243
Test: atest systemfeatures-gen-tests systemfeatures-gen-golden-tests
Flag: EXEMPT tool only change
Change-Id: I7d89106af2fe447e79ef8149b83741fcb371f7d3