Add genrules for merged current.txt

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