TestableContext: override finished in TestWatcher

This is called on either test outcome, so we can deduplicate the code.

Bug: N/A
Test: $ atest --no-group-test \
    InputTests:com.android.server.input.debug.TouchpadDebugViewTest \
    InputTests:com.android.test.input.AnrTest
Flag: TEST_ONLY
Change-Id: Ibf4bfa489bd704acf00e8fcc0ff0a1f90538a511