Bluetooth: do not dimiss fragment when Activity.finish() is called

* When Activity.finish() is called, it's associtated fragments are all
  dismissed automatically
* Cached used fragments are dimissed in onCreate() before new ones are
  created

Bug: 62230203
Test: Pair with Bluetooth device, Settings unit tests
Change-Id: Ieca88ba0660c5407f0d88d572d06a722c642ac39