Change a11y step for the SeekBar in VolumeDialogImpl

The problem is that SeekBar splits the whole range into a hardcoded 20
steps when adjusting it with the voice overs turned on. This doesn't
align with the volume levels which triggers weirds round ups/downs.

Flag: n/a
Test: atest UtilTest
Test: manual with the voiceover
Fixes: 320608907
Change-Id: I4326e276d40c302e8de526437a0411657a76d0fc