le_audio: Handle pause and resume of local broadcast

Handling broadcast source add/remove while local broadcast is
resumed/paused give receivers more clear context on how to behave. Leaving
not changed receiver, leave receivers a synchronization decision which may
be not as expected by broadcaster, especially when there will be a uniast
handover e.g. trying to sync to PA/BIS when there is preparing Unicast
streaming.

Bug: 298079825
Tag: #feature
Test: atest BluetoothLeBroadcastMetadataExtTest
Change-Id: I26b8783589a9c6e6d9cdf03c473310634b06b892