draw filtered if dragging but not moving after a 100ms delay

Some artifacts show up when dragging, but since the screen is in
motion, they're usually go unnoticed. If one quits moving, the
artifacts are more visible.

Some devices generate fractional movement data while the
screen is touched and held steady; others do not. Also, mouse-based
implementations will not generate random fractional movements. If
the device generates movements, invalidate the screen to transition
once from unfiltered to filtered drawing.

For the case that the device does not generate movements, fire a
timer when the screen is drawn and a drag is in progress to detect
that there is no drag motion.

The timer fires if the dragged screen is unmoved for 100 ms, then
invalidates so the next redraw uses filtering to remove the artifacts,
and starts a continuous timer to keep the scrollbar awake until
the drag ends or there is more drag motion.

When detecting that there is no effective movement on a noisy device,
use the original fractional deltas instead of the integer ones to
detect sub 1 pixel differences.

This was tested on the emulator (no noise) and a sapphire (noise)
to make sure that events load the queue only when required.

fixes http://b/issue?id=2092446

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