Remove _API_FILE variables

There are no known usages of these variables, and many have been
incorrectly set for a long time.

Also convert the <module> phony for api txt to androidmk.

Bug: 152479829
Test: m
Change-Id: Iff355041d88eb59ac47dd19a03e993cf1ce3927e
Merged-In: Iff355041d88eb59ac47dd19a03e993cf1ce3927e