check-flagged-apis: handle nested flags

Consider

  @FlaggedApi(FLAG_OUTER) Clazz {
      @FlaggedApi(FLAG_INNER) method();
  }

If FLAG_OUTER is disabled, any class members are ignored. Teach
check-flagged-apis to recognize this and stop reporting false positives.

Bug: 339183637
Test: atest --host check-flagged-apis-test
Change-Id: Ie6799e952dc33874c1239231f841d7dfd947c7ce