Handle connection request to already connected device.

Use case: Test the connect collision scenario from DUT and Headset.

STR:
1. Create a connection from DUT to headset, disconnect and unpair the link.
2. Turn OFF and turn ON headset, keep in pairing mode, headset keeps trying
   to connect back to DUT.
3. Inquiry in DUT, and connect to headset.

Failure: DUT gets stuck in pairing, whenever there is a clash in
accept_connection and create_connection.

Root cause: When DUT and remote try to connect to each other simultaneously,
an ACL handle is created with first connect request and the second connect
request is also processed.DUT doesn't check whether there is already a
ACL handle present between the devices.

Fix: Fixed the issue by reusing the existing handle and
discarding the second connect request.

Bug: 21896912
Change-Id: I2001bc3b2fa50f827a4e3b41cde91af1ea106c54