check-flagged-apis: parse API signature files

Teach check-flagged-apis to extract flagged APIs from API signature files.

To keep things simple, only consider fields for now: support for classes
and methods will be added in a later CL.

Note: `m frameworks-base-api-current.txt` will generate an API signature
file that includes both the platform and mainline APIs.

Bug: 334870672
Test: atest --host check-flagged-apis-test
Test: check-flagged-apis --api-signature out/target/product/mainline_x86/obj/ETC/frameworks-base-api-current.txt_intermediates/frameworks-base-api-current.txt
Change-Id: Ic244b896672569f44af793796189b34c1f9d0c36