Report UnflaggedApi issues as warnings but treat as errors when new

The `UnflaggedApi` issue is only reported when doing API linting and
defaults to `Severity.HIDDEN` because AndroidX (among other users) do
not support using `@FlaggedApi` annotations. This change switches it to
be warning by default in the  Android build as there are some existing
instances of it, but will be treated as an error by the "Build Warnings
Aye Aye Analyzer" in a Gerrit change if it occurs on lines that are
modified/added by that change.

Bug: 361582214
Test: m checkapi
Change-Id: Ie5ed23d00dd49454aeb5b43ccd8ad027d9d34fd2