Bluetooth: Not allow Blank BT device name.

Add empty string check for BT device name
and not enable ok button until entered
device name is not empty.

Change-Id: Ib7c460bee5c8d86d13acd66d228279dd1a240c25
CRs-fixed: 471158
Merge "Bluetooth: Not allow Blank BT device name." into jb4.3