Fix: some flagged APIs are not recorded.

* The @FlaggedApi annotation can be used for classes. When a class is
  annotated but its APIs are not, they should be counted as flagged.
* Class constructors are also APIs.

Bug: 331294167
Test: manual test `extract-flagged-apis <input> <output>`
Change-Id: Ic6d25eb59faa381f033751557343ad85c1602e41