Fix bug 5159596 - Slider grabs touch point when trying to scroll a list

SeekBar widgets were not good citizens when placed in scrolling containers.
Detect when a SeekBar being touched is in a scrolling container and fall
back to a touch slop-based method for beginning a drag. Taps along the
seek bar itself will still change the value on touch up, even if the touch
slop never initiated a proper drag.

Factor out the code used to determine scrolling container status in View
into something that can be reused elsewhere in the framework; leave it
@hidden for now.

Change-Id: I59a21e8ab2c3b0092fafedbcd6ae1ee15f45971c