proximity: Add new Link Loss service
proximity: Implement Link Loss write callback
gatt: Do not report an error twice

If an error is reported by btd_service_connecting_complete() it shouldn't
be also reported by the profile .connect() function.
gatt: Fix leaking a btd_service

We should only keep track of btd_service when we have an active
connection.
gatt: Allow to cancel connections
proximity: Move old reporter profiles to reporter-legacy
proximity: Add support for tracking the connection state
proximity: "Alert" the user when the connection drops
**** Squash to  proximity: Implement Link Loss write callback
proximity: Implemented Link Loss read callback
proximity: Add support to read/write link loss to reporter
proximity-api: Improve description for ImmediateAlertLevel behavior