Support chorded fallback keys.

Also be more careful about canceling fallback keys during focus
transitions, when the application handles the key, or when the
policy decides to do something different.

Fixed a crash due to JNI CallObjectMethod returning an undefined
value (not null) when an exception is thrown.

Fixed a crash due to the policy trying to create a Dialog for
recent apps on the dispatcher thread.  It should happen on the
policy's Looper instead.

Bug: 4187302
Change-Id: I659a3fd1bd2325ed36d965f9beb75dacb89790c9