VolumeControl: Improve the volume broadcast flow

Follow up CL to improve some logic in aosp/2880666, move volume update
out of loop.
No-op change and the function is behind feature flag.

Bug: 314738219
Bug: 307408418
Test: atest VolumeControlServiceTest
Change-Id: Ie9c30d9c59ae910f770479e8644f565204102f1b