Allow enabling Bluetooth without auto-connecting.

This is a feature used for NFC-to-Bluetooth handover:
we want to enable BT for file transfer, and disconnect
it when we're done. During this period we don't want
to auto-connect other devices - it should be transparent
to the user that Bluetooth is used. Also, don't allow
A2DP/HSP incoming connections.

Change-Id: I0a03e8084c439b1271b6a80f4d9da5aacfe19c45