Don't preserve the confusing names from make. Rename AllCodenames to
ActiveCodenames, add FutureCodenames, and add CombinedCodenames to
fetch the *real* AllCodenames.
To allow `introduced=P` in NDK library definitions, we need to know
that P exists. Use the combined list of current and future API
codenames generating stubs.
Test: make ndk
Test: check out/soong/api_levels.json
Bug: None
Change-Id: I435f9ce7446236edc268a84e33474044a55a6302