Add android.test.mock.stubs-system

The android.test.mock package contains one class with some methods that
are annotated with @SystemApi which are not present in the
android.test.mock.stubs. This adds a new android.test.mock.stubs-system
library that includes those additional methods.

Checked that the new API definition files contain the exact same
definitions for the android.test.mock packages as are found in the
api/system-current.txt and api/system-removed.txt files.

Bug: 30188076
Test: make update-api && make checkbuild
Change-Id: I4644d30716e556204caa4acb53d7af84ca520271