le_audio: Unify local audio source notification usage

Notify local audio source has common usage as local audio sink, let them
both have the same helper method for better readability.

Bug: 360051130
Test: atest bluetooth_le_audio_client_test
Flag: EXEMPT; simple fix
Change-Id: I4c4f2e4d746ab185c2d7a8e1c3043c8ef9bb1224