We are also lacking appsearch, but it doesn't yet have api txt files.
Bug: 169034951
Bug: 169304493
Test: diff current.txt with generated txts
Change-Id: I0866605f0b4ba450ae824af00d24502fda060a5f
Feed metalava all module txt files and write a single file. These are
intended to replace the checked in monolithic current.txt eventually.
Note that they are unused for now, they're getting checked in to enable
easier collaboration.
Bug: 169034951
Test: m frameworks-base-api-{,system-,module-lib-}current-merged.txt
Test: Manual inspection for now, some very minor diffs
Change-Id: I1e1d0aff8475d3aaa354308bb70d54201488de61