APIs in a nested class can be flagged by outer class.

If a class is not an inner class, use its @FlaggedApi annotation value.
Otherwise, use the flag value of the closest outer class that is annotated by
@FlaggedApi.

Bug: 331294167
Test: atest extract-flagged-apis-test
Change-Id: I9d40d3e7c5065a2a737d5420c4235445c6d16654