Disable duplicate back handling when a BackInvokedCallback is already registered

Whenever Lockscreen or Shade/QS are visible, we register a callback with back action handling to OnBackInvokedDispatcher. Similarly, whenever we receive a KEYCODE_BACK action (such as the TalkBack back swipe gesture), we call the same back action handling logic. This meant that whenever we were on Lockscreen/Shade, we were always handling back actions twice when we received KEYCODE_BACK.

Bug: 381193919
Test: verified with logging that back handling only triggers once over LS/Shade/QS
Flag: EXEMPT bugfix
Change-Id: Ic92bbb82561a20734a9fb92fa12804d37e4766be