record-finalized-flags: use metalava's flag-report as source of truth

Replace the logic to grep through signature files looking for
@FlaggedApi annotations with reading the flag report CSV file generated
by metalava.

Bug: 404452062
Test: atest record-finalized-flags-test
Test: m out/soong/.intermediates/finalized-flags.txt && head out/soong/.intermediates/finalized-flags.txt
Change-Id: Ieca65ddee8a0b6eefbfdcab69614eece6dae4247