Reintroduce "Deprecated at Birth" linter.

Earlier this year the "apilint.py" script was removed from the build,
since it's been replaced by Metalava.  However, several features
haven't been ported yet, so this change revives them to support the
SDK finalization process.

This change also updates the script to handle "Signature format: 2.0"
files, and reads the API surface from a directory which contains
several ".txt" files to support the new Mainline API structuring.

Bug: 189224267
Test: manual
Change-Id: Ifc4c24a7e159db6725897800d67947d1fd8b9880