Remove volume slider snapping in volume dialog

We've received the feedback that the snapping slider feels junky and
like there are missing frames in the animation. Hence removing it and
making the slider behaviour more like the previous dialog had.

Flag: com.android.systemui.volume_redesign
Fixes: 406268223
Test: manual on the foldable. Adjust volume and observe that is doesn't
snap to the next value
Test: atest VolumeDialogScreenshotTest

Change-Id: I3338d0f5eccb3c825dbaa35c6cc43c141ce16c5a