LeAudioService: Minor changes to store volume for the group

This patch address review comments for the patch which is already merged
in internal branch.

Bug: 233975367
Test: atest BluetoothInstrumentationTests
Tag: #feature
Merged-In: I26b4deed716610b98cfd08cfd2ac43fd0ecfab65
Change-Id: I26b4deed716610b98cfd08cfd2ac43fd0ecfab65
(cherry picked from commit dc7f9e72)