Improve connection handling logic in ConnService

Connection for HFP HF role in Bluetooth is currently trying to
handle all cases of connected vs disconnect HFP. This can be offloaded
to ConnServ where the connection lifecycle should be maintained (and not
in the connection itself).

1. Fixes NPEs generated by the BtCarHfpFuzzTest.
2. Reduces the number of calls to AT+CLCC

Change-Id: Ibccadb7721cbbd2260cbfc5685d66027277e8a59
(cherry picked from commit 1ff2e6e9306272bd113c8633c9b52fb169eddcb3)