LeAudioService: Store volume for the group

With this patch, if volume was previously known for the group,
it will be used when device become Active again

Bug: 233975367
Test: atest BluetoothInstrumentationTests
Tag: #feature
Change-Id: Ic1fae47b4e94975dffc69ee27f641c64cbf0df61
Merged-In: Ic1fae47b4e94975dffc69ee27f641c64cbf0df61
    (cherry picked from commit 90b1b642cd5557364d3045da448e7d42560e2b2c)