SetActivePeer should only update the active peer

Problem: While creating a peer, FindOrCreatePeer assigns the
active peer if there is no active peer device assigned. As a
result SetActivePeer results in a no-op.

Solution: FindOrCreatePeer should not assign the active peer
until the device is connected. Once AVDTP connection is connected,
then only the active peer should be updated.

Test: m seahawk-userdebug.
Test: atest bluetooth_test_gd_unit
Bug: 305110114
Change-Id: I53b22aa7dcad65be9ecb6ad45fe5df88bf4739a6