Fix a system crash due to orphan HandlerThreads

In SeekBarVolumizer class a HandlerThread is
instantiated that is not halted after the object
is garbage collected. The final consumer of
this class is volumes adjustment dialog in Settings.
If the user invoke the dialog several times, many pipes
are opened in Settings process that it finally exceeds
the maximum permitted number and the system crashes.

Change-Id: Ifd777365b5779f5db92c605413eb0254a53c8b00