Fix multiple instances of BT file transfer ongoing dialog

If during multiple BT file transfer dialog is displayed
several times from status bar, then it takes as many
taps to cancel the transfer.

* Add Intent.FLAG_ACTIVITY_CLEAR_TOP flag to clear previous
  poped up dialog.
* Unregister the Content Observer whenever the file
  transfer completes.

Bug: 28872363
Test: manual
Change-Id: I6bffe28bcd6973ddf52cfff063961e1981d139dc