Experiment with allowing tap to break through to interact with the PIP.

Test: Enable SysUI tuner, tap once on PIP to interact with the activity.
      This is only experimental behaviour, and
      android.server.cts.ActivityManagerPinnedStackTests will be updated
      accordingly if we keep this behavior.

Change-Id: I278ab8c360c44718cfcac0fd761f476a875f9b15