Move all of the output files of the metalava rule into a single
subdirectory to make it compatible with sandboxing in sbox.
Test: TestDroidstubs
Change-Id: I66101c0c224dee702c8175e61c12cc9cd1aa8b93
Split part of droiddoc.go into droidstubs.go. Also split droiddoc_test.go
and droidstubs_test.go out of java_test.go.
Test: go test ./java
Change-Id: Iea742e75b6925b135016f7bbf3a168c696a6c433