Update PiP AppOps flag to enforce all PiP functionality.

- Previously it only enforced whether an app could enter PiP as it was
  hiding.
- Also updating metrics constant name to reflect the change.
- Adding public OPSTR for apps to check if it the user has disabled PiP

Bug: 35957404
Test: android.server.cts.ActivityManagerPinnedStackTests
Change-Id: I979b5c6d4c85946623999ced1cfb197d830ccecb