Implement NFC-BT handover for Bluetooth headsets, such as Nokia BH-505.

Touch once to turn BT on, pair, connect HFP and A2DP, and start the music.
Touch again to disconnect HFP and A2DP.

Change-Id: Icfe120606aae5e80b04cc4aba3b03331a1213676
TODO: enable Bluetooth without causing auto-connection to *other* devices
TOOD: disable Bluetooth when disconnecting if it was enabled for this device
TODO: il8n / UI review
TODO: check security issues around auto-on BT and auto-pair