LeAudio: Improve setting active Le Audio group

Make use of new API to get group lead devices.
With this patch, we are making sure, that Telecom will try to set very
same active device for the given group as LeAudioService.

Tag: #feature
Bug: 159786353
Sponsor: jpawlowski@
test: Manual use Dialer list of active devices
test: atest TelecomUnitTests

Change-Id: Ifcfba8355fdd14cc4a3f763c4013d2e4c8ea33df