check-flagged-apis: record interfaces when parsing classes

Extend ClassSymbol with a list of the interfaces that class implements.
This will be used in a follow-up CL to improve the logic that checks if
a class member exists in the api-versions.xml data.

Bug: 334870672
Test: atest --host check-flagged-apis-test
Change-Id: I4db7ff47c3ce40ca892cb872810dd559426dfcb8