audio: change volume only if effect updates the volume

Incase of offload effects, setting unity gain leads to
full volume on enabling effects, as unity gain (volume
1.0f, 1.0f is set to audiohal).

Don't set volume always to unity gain, instead
check if effect interface has updated the volume
then only, change the value.

Bug: 296954124
Test: Start a offload effect and check volume.

Change-Id: I0c20e1235a90b77c0d6ad8fba586cdf006f37224