Unregister display listener

Display listener must be unregistered in ScanManager::cleanup()
to avoid getting display state change callbacks between
ScanManager::cleanup() and ScanManager object destruction.

BUG: 64253576
Test: Screen on and screen off during BLE scanning.
      Screen on and screen off with Bluetooth off.

Change-Id: I5b1f26696c44aaaca3720a200d85983afc8d2759