Add a11y setting for Automatically click after pointer stops

Adds new preference screen for the feature, where the feature can be
enabled/disabled, and delay before click can be specified (implemented
as seek bar preference, enabled iff the feature is enabled).

Preference is added to Accessibility setting. The preference summary is
set to string describing the delay after pointer stops moving before
click is scheduled (the string also contains numerical value of the
delay).

BUG=20049245

Change-Id: Ib7b2f5c24efbd08e3200143c069bcdbc9b5a327a