Bluetooth-OPP: Handle incoming file confirmation pop-up

While BT is turned OFF during ongoing OPP Rx, DUT is not able display
incoming file confirmation pop-up after BT is turned ON again. This
is because not able to start the notification thread properly after
BT is turned ON.

This change will handle incoming file confirmation pop-up.

CRs-Fixed: 594171
Change-Id: I12950b621eddf2806c0b0f1d9c9ac17b02b4e9b8