record-finalized-flags: add actual implementation

From the command's --help text:

---- 8< ----
The prebuilts/sdk/<version>/finalized-flags.txt files list all aconfig flags that have been used
with @FlaggedApi annotations on APIs that have been finalized. These files are used to prevent
flags from being re-used for new, unfinalized, APIs, and by the aconfig code generation.

This tool works as follows:

  - Read API signature files from source tree (*current.txt files) [--api-signature-file]
  - Read the current aconfig flag values from source tree [--flag-file]
  - Read the previous finalized-flags.txt files from prebuilts/sdk [--finalized-flag-file]
  - Extract the flags slated for API finalization by scanning through the API signature files for
    flags that are ENABLED and READ_ONLY
  - Merge the found flags with the recorded flags from previos API finalizations
  - Print the set of flags to stdout
---- >8 ----

Bug: 377676163
Test: atest record-finalized-flags-test
Merged-In: Icde8c63fc54791429865168989bfb6af01845d15
Change-Id: Icde8c63fc54791429865168989bfb6af01845d15