check-flagged-apis: add support for methods (no parameters)

Teach check-flagged-apis to parse methods. The implementation is only
half done: method signatures that accept parameters are ignored. A
follow-up CL will add support for these.

check-flagged-apis treats constructors and regular methods the same.

Bug: 334870672
Test: atest --host check-flagged-apis-test
Change-Id: Ie98db767289ac2a35aa85371f60ecb3970170d86