Refinements to magnification for improved wearable support.

This change refactors ScreenMagnifier to use resources for its triple-tap
adjustment and scale threshold values.  New values more appropriate for
wearable form factors are supplied.  This also fixes a bug in the triple-
tap detection logic where the incorrect ViewConfiguration value for the
tap threshold was used, prematurely disqualifying some touch events as
potential taps.

Change-Id: If47e556aadb5beb1bad24644122560c6fbe33bad