Implement native key pre-dispatching to IMEs.

This significantly re-works the native key dispatching code to
allow events to be pre-dispatched to the current IME before
being processed by native code.  It introduces one new public
API, which must be called after retrieving an event if the app
wishes for it to be pre-dispatched.

Currently the native code will only do pre-dispatching of
system keys, to avoid significant overhead for gaming input.
This should be improved to be smarted, filtering for only
keys that the IME is interested in.  Unfortunately IMEs don't
currently provide this information. :p

Change-Id: Ic1c7aeec8b348164957f2cd88119eb5bd85c2a9f