Add android.test.runner.stubs dependency

In preparation for the removal of the non-junit classes in the
android.test.runner library from the android.jar this adds a dependency
on android.test.runner.stubs to ensure this code will continue to
compile.

The following change descriptions were generated automatically and so
may be a little repetitive. They are provided to give the reviewer
enough information to check the comments match what has actually been
changed and check the reasoning behind the changes.

* tests/Android.mk
    Added 'android.test.runner.stubs' to LOCAL_JAVA_LIBRARIES because
    ContactsTests's will needs them in order to compile once its
    classes are removed from the current SDK on which it currently
    depends.

Bug: 30188076
Test: make checkbuild
Change-Id: Id461d950eb755237676c135275f5d84ad81487b5