Add validation for publicAlternatives

Validate all the {@link ...} tags in the publicAlternatives field of
@UnsupportedAppUsage annotations, by cross-referencing them against the
current public API.

Test: atest class2greylisttest
Bug: 130721457
Change-Id: Ic3984687cb7ce323d431975767f9ba6b4ef7b49b
19 files changed