Properly handle outgoing connection requests in opening/opened state

This happens in race conditions when we try to connect to a remote
A2DP device from Bluetooth Settings UI and simultaneously there is incoming
connection request from previously connected remote. This fix addresses
the scenario where because of first A2DP connection btif moves to opening/
opened state and then the second connect request is received.
This scenario is handled by gracefully rejecting the second connect request
from btif layer if it is already connected or about to be connected by the
first connect request.

Bug: 22028876
Change-Id: I53be400d1d7c9a84ca9d9a75b6b6f010975cf0cf