check-flagged-apis: parse flag names and values

Teach check-flagged-apis to parse the parsed_flags protobuf generated by
aconfig.

Note: `m all_aconfig_declarations` generates a protobuf file that
contains all info about all flags.

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 --flag-values out/soong/.intermediates/all_aconfig_declarations.pb
Change-Id: I397b32ae2a373b429ef6ce22e0a06a0f15202b91