Suppress R8 warnings for support library annotations

Most of these annotations are stripped at dex compile-time, and are not
necessarily present in the generated dex library's classpath. As the
classpath is used to inform R8 about `libraryjars`, this can cause
reference failures during R8 execution when warnings are enabled.

For now, suppress the reference warnings. Unrecognized runtime-visible
annotations will still be kept, and this should unblock efforts to
re-enable R8 warnings for many platform targets. A proper solution will
be to ensure the appropriate annotation libs are fed as `-libraryjars`
to R8.

Bug: 242088131
Test: m + `optimize { ignore_warnings: false }`
Change-Id: Ie92db2a9725ff520b311c6b66a50feae79463925