Generate genrule modules for merged api txts

Add a soong plugin that can generate the genrule definitions for the
merged api txt files. This will allow removing a lot of duplicated
lists in the Android.bp and simplify adding new modules.

Bug: 169103987
Test: follow-up that uses these rules (branch dependent)
Change-Id: I0c7a9921b7f4c5b74f8b652cb357abf9444ce93f