Add input filter mechanism for accessibility.

This patch adds a mechanism for capturing, filtering, transforming
and injecting input events at a very low level before the input
dispatcher attempts to deliver them to applications.  At this time,
the mechanism is only intended to be used by the accessibility
system to implement built-in system-level accessibility affordances.

The accessibility input filter is currently just a stub.
It logs the input events receives and reinjects them unchanged,
except that it transforms KEYCODE_Q into KEYCODE_Z.

Currently, the accessibility input filter is installed whenever
accessibility is enabled.  We'll probably want to change that
so it only enables the input filter when a screen reader is
installed and we want touch exploration.

Change-Id: I35764fdf75522b69d09ebd78c9766eb7593c1afe