Fix http://b/issue?id=2098873 (drawing glitch in SeekBar).

The underlying ProgressBar implementation doesn't know how
to correctly update the thumb graphic, so AbsSeekBar must
call invalidate() at key moments to ensure things look
correct.  This bug is one such instance: when you pick your
finger up off the seekbar, the "value" of the progressbar
appears not to change, so ProgressBar feels no compunction
to repaint. AbsSeekBar now invalidate()s on ACTION_UP and
ACTION_CANCEL to ensure that the thumb is correctly drawn.