Add genrule to find API file Metalava used to generate annotations

The API file Metalava used to migrate NULLs would be
27.txt(PLATFORM_SDK_VERSION - 1) As tnorbye@ mentioned. So add a genrule to
fetch the API file for annotations.

Test: m -j metalava-api-stubs-docs
Bug: b/78245848
Change-Id: I4194d34a6bb6ab0aeccc0b39e6fbb0885c2d03c3
1 file changed