Modify the calculation method of the volume DB value.

When the volume range is 0~100 and the UI adjusts the volume to level 1,
the volume DB value calculated by the volume curve is VOLUME_MIN_DB. The
volume value obtained after DbToAmpl conversion is 0.0f, so the music is
silent.

Test: Play music at volume level 1.
Bug: 280972677
Change-Id: I1cab6ff7a28c8da03564812d76bc9be9c4acf295