Lower touch slop in ViewConfiguration.

Our typical touchscreens are now good enough that we don't need the
touch slop to be as large as it once was. Lower it to increase
responsiveness while scrolling.

Move the touch slop constant to the config resource
config_viewConfigurationTouchSlop so that it may be more easily
tweaked in device-specific overlays.

Change-Id: I7c11d8affcb98e91654203beb13a26cfeec18f7c