Add CtsVerifier test for Screen Pinning

Some lock task functionality is tested through xts, however this does
not cover the user based portions of screen pinning, such as user
being able to exit anytime through back+overview.  This adds a quick
check of the API and that the user is still able to exit.

Bug: 18992713
Change-Id: I04017c76308c9dfd5b48b123cb398bb5a4c29509