VolumePanel: volume bar disorderly jump when drag it.

under stream type is STREAM_BLUETOOTH_SCO or STREAM_VOICE_CALL,
if we drag the volume bar, it will disorderly jump.

for example, connect bt headset and make a call, change volume and
drag volume bar during call, this isue will appear.

Change-Id: Ie61d6010c3eb249448878533250c7fad0e24f089