check-flagged-apis: parse API versions XML

Teach check-flagged-apis to parse API versions XML; this represents the
APIs after metalava has processed the source and kept APIs as is, or
reverted them to the previous SDK snapshot, according to their
@FlaggedApi flags.

As with the API signature parser, limit support to fields to keep things
simple; support for classes and methods will be added in later CLs.

Note: `m sdk dist` will generate an API versions XML file.

Bug: 334870672
Test: atest --host check-flagged-apis-test
Test: check-flagged-apis --api-signature out/target/product/mainline_x86/obj/ETC/frameworks-base-api-current.txt_intermediates/frameworks-base-api-current.txt --flag-values out/soong/.intermediates/all_aconfig_declarations.pb --api-versions out/dist/data/api-versions.xml
Change-Id: I779a0d0cdb8a50536d3fc8d517fa38ba4b0dcd1c