check-flagged-apis: consider interfaces when looking up symbol

When searching for potential errors, if a symbol can't be found in the
api-verions.xml data, check if it is present in any of the class'
interfaces.

A follow-up CL will add similar logic to handle super classes.

Bug: 334870672
Test: atest --host check-flagged-apis-test
Change-Id: Ia6dfcfa8495b89465db60f6a4eb77d304112046b