init: start move to Android.bp

Move libinit, init_tests, and test_service to Android.bp
Leave init in Android.mk as it has unfulfilled dependencies, but
create a comment in Android.bp for future use.
Remove libinit_parser and init_parser_tests as that code was never
used in init.

Bug: 36970783
Bug: 37512442
Test: Build, boot bullhead, init unit tests
Change-Id: Id81cd10ea09453a5fd762ba9189276aad79d5444