Bluetooth-OPP:ANR fix while sending 250 files

ANR is observed in OppLauncherActivity while sending 250 files
because OppLauncherActivity thread start DevicePickerActivity,
but in this case user clicked back key before PickerActivity starts
and then user tries to send 250 files again. At this point one of
the DevicePickerActivity got blocked.

CRs-Fixed: 710599
Change-Id: I3df3bc8559e91df6360fbc55d8e33d567184fe51