Make android.test.mock.stubs in Android.bp

Stubs libraries android.test.mock.stubs and
android.test.mock.stubs-system are now built in Android.bp.
(Note that API check and update are still done in Android.mk)

As a good side effect, this allows android.test.runner[-minus-junit]
libraries (that are defined in Android.bp) to be built with the
android.test.mock.stubs instead of the runtime library
android.test.mock. This in turn allows us to prevent java:sdk ->
java:platform dependency also in Soong, because the runner libraries
were the only violators.

Bug: 73829976
Test: m -j android.test.mock.stubs android.test.mock.stubs-system
Test: m -j checkapi
Test: m -j update-api
Change-Id: I830a8eb13aa43b9aa4c8394f3c2fd91ee18b1f85