First stab at attaching native event dispatching.

Provides the basic infrastructure for a
NativeActivity's native code to get an object representing
its event stream that can be used to read input events.

Still work to do, probably some API changes, and reasonable
default key handling (so that for example back will still
work).

Change-Id: I6db891bc35dc9683181d7708eaed552b955a077e