Feed mouse and keyboard events to EventStreamTransformation

Updates AccessibilityInputFilter to pass keyboard and mouse events
through enabed EventStreamTransformations.

This is done in preparation for implementing Autoclick on mouse stop
feature, which depends on those events.

Existing EventStreamTransformation implementstions are modified to
non touchscreen motion events and key board events.

Adds stub EventStreamTransformation (AutoclickController) that will
be used to implement autoclick feature.

Implements key event filtering as a separate EventStreamTransformation
(instead of doing it directly in AccessibilityInputFilter).

Introduces private EventStreamState classes to AccessibilityInputFilter,
which keep track of whether event sequences for different input sources
have started, and help reduce code duplication when determining whether
an event should be fed to registered EventStreamTransformers.

BUG=23113412

Change-Id: If115799bbe4debce48689370ff5ea9fa6dab9639