Add android.test.* api files

API txt files for android.test.* are added. The contents are empty as
they didn't exist before API level 28.

android.test.mock.stubs will be generated by soong, so delete the
Android.bp for android.test.mock.stubs

Bug: 77577799
Test: make -j
      mkae checkapi
      make update-api

Change-Id: I4ad641018a45b3a5fef0338fc7fda65800be7b37
22 files changed