p2p: Display P2P peer MAC address if device name is empty.

Display MAC address of a P2P peer in an invitation prompt
message on receiving invitation request from the P2P peer
with empty device name.

Change-Id: Icabeb91d666437746eb5a733ec582849eb7295a5
CRs-Fixed: 506109
Merge "p2p: Display P2P peer MAC address if device name is empty." into du44