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

include/ui/Input.h

@@ -128,6 +128,12 @@ enum {
     // input device or an application with system-wide event injection permission.
     POLICY_FLAG_TRUSTED = 0x02000000,
 
+    // Indicates that the input event has passed through an input filter.
+    POLICY_FLAG_FILTERED = 0x04000000,
+
+    // Disables automatic key repeating behavior.
+    POLICY_FLAG_DISABLE_KEY_REPEAT = 0x08000000,
+
     /* These flags are set by the input reader policy as it intercepts each event. */
 
     // Indicates that the screen was off when the event was received and the event