Adding experiment for minimized pinned stack.

- Also refactoring the PIP touch handling to be independent gestures

Test: Enable the setting in SystemUI tuner, then drag the PIP slightly
      offscreen. This is only experimental behaviour, and
      android.server.cts.ActivityManagerPinnedStackTests will be updated
      accordingly if we keep this behavior.

Change-Id: I5834971fcbbb127526339e764e7d76b5d22d4707