Stubs/api check for legacy-test and android.test.runner

Adds stub generation for legacy-test classes and android.test.runner
classes. Changes name of android.test.mock.sdk to
android.test.mock.stubs for consistency and clarity, adds new
temporary android.test.mock.sdk target to prevent build breakage.

Adds api check and update targets for legacy.test.stubs,
android.test.runner.stubs and android.test.mock.stubs to prevent
accidental changes to the API.

Bug: 30188076
Test: make checkapi and make update-api
Change-Id: I40cc7b967f1ab1c7e148407d6965526550ee2f9b