Cache last focussed display id in LocalBluetoothManager

This is used by Bluetooth paring dialog to make sure it opens on external display when triggered from external display

Bug: 416872340
Flag: EXEMPT, usage of the new method to query the cached display id will be flag guard
Test: manually tested that the dialog opens on external display when triggered from external display

Change-Id: I6976954e018b4013bc3886c67e151fe54c1e3b1b