le_audio: Handle overlapping connected LE Audio devices

While doing handover there were a possible moment that none of LE Audio
devices were connected. This could lead Audio Manager to pick primary
device. Such situation cause bad user experience e.g. playing music on
phone speaker for a while.

Tag: #feature
Bug: 307703030
Bug: 308171251
Test: atest LeAudioServiceTest
Change-Id: I4875a13c80fae8f5df3b889733c4fab4a8a98d1c