LeAudioService: stop background scan when all devices are connected

Without this patch, we would keep doing background scan even when all
known devices are connected.

Checking just if state machine is connected is unreliable - lead device state machine after disconnection would still claim to be connected, but the acl is indeed disconnected. Therefore, addition of mAclConnected was needed.

Test: atest LeAudioServiceTest
Bug: 301520529
Change-Id: Ie5712f19487ff9ae2df1d329cd3de37f07cb4a6f