Bluetooth-OPP: Handle nullpointer exception in IncomingfileActivity

When IncomingFileActivity is timed out, timeout is called which
tries to dereference mContentView object. But in L-release this
object is not created and while trying to dereference this object cause
NullPointerException and that leads to BT application crash.

CRs-Fixed: 740959
Change-Id: Iada201f8f8c96af93e4f5179c4106fd164de4599