Introduce settings for Automatic click when mouse pointer stops

Settings in question are:
- accessibility_autoclick_enabled -> Whether the feature is enabled
- accessibility_autoclick_delay -> Amount of time mouse pointer has to
    stay still before performing click

Enable/disabled AutoclickController event stream transformation (which
implements the feature) based on the accessibiilty_autoclick_enabled
value

BUG=23113412

Change-Id: I8178d10c09d86713bb4c725c95268a7c5f432eb4