Add compat checking of the merged APIs

Compare the current merged txt to the last one checked into
prebuilts/sdk. For the system and module-lib APIs, also feed
the current public API in as the "base" API.

Bug: 174847574
Test: m frameworks-base-api-{,system-,module-lib-}current-compat
Change-Id: I7d4274ec1561be131c4f0298b49c05b24f03cb06