Fix trackball interpretation of precision.

The trackball to dpad synthesis was using the MotionEvent's precision
field to determine a threshold for movement but the calculations
involved did not actually make sense for any value of precision
less than 2.0.  This worked fine before since the InputReader
hardcodes the trackball's precision to 6.

Injected trackball events may have a different precision which can
result in the thresholds being set inappropriately.  For example,
it was not possible to move focus by one unit at a time when
the precision was set to 1.0.

The old code was probably using precision as a way to set a
threshold based on the trackball moving by some minimum number
of physical ticks, in this case 2.  But the code will work just
as well if we set an absolute threshold based on distance
traveled given that the input system is already expected to
normalize the trackball movements before delivering them to the
application.

So stop using precision.

Bug: 8473020
Change-Id: I3c6f7fb1b507f8cf5608b47550e7345fea3352fa