Cleaning and new constant in ViewConfiguration

Fixed the documentation and comments for dips versus pixels.

Added a new DOUBLE_TAP_TOUCH_SLOP to replace the hard-coded and
*unscaled* value used in GestureDetector. Assuming this value was
ok for hdpi devices, set it to 20 / 1.5 ~= 16 in dips.

New method kept hidden to keep API unchanged.

Change-Id: I6d237faa2fdb7714e0e3558978af130df9a061a5