le_audio: Use broadcast device for broadcasting usecase

With this patch, Audio Frameworks is notified about Broadcast device to
be connected for broadcasting usecase.

Tag: #feature
Test: atest LeAudioServiceTest
Sponsor: jpawlowski@
Bug: 210986334
Ignore-AOSP-First: no AIDL in AOSP

Change-Id: I69324802bddb5eeb05bdc8ccf7067d445a48026a