check-flagged-apis: add more details to Symbol class

Change Symbol from a wrapper around a String to a more fleshed out data
class; symbols now encode if they represent a class, or a class member
(including a reference to the containing class).

Bug: 334870672
Test: atest --host check-flagged-apis-test
Test: croot && ./build/tools/check-flagged-apis/check-flagged-apis.sh # with and without this CL; the output should be the same
Change-Id: I003535c721c45d559d00fb3e008325e1db0e18c0