Introduce InputConnection#getHandler().

Currently there is an internal hidden class named
ControlledInputConnectionWrapper which works as a proxy in the
application process to receive incoming binder calls from input method
and dispatch those method calls again on an appropriate thread.
Although this is a kind of implementation details, basically you can see
the same design everywhere in the Android.

Currently ControlledInputConnectionWrapper is initialized with
view.getHandler(), where the view here is the View which was used to
call View#onCreateInputConnection().  This is actually a reasonable
behavior because we have generally assumed that there the only
reasonable way to implement InputConnection is to extend
BaseInputConnection, which is designed to be able to work only on the
UI-thread associated with the target view.

However, on Android N and onward, we are going to ensure that
BaseInputConnection can be re-implemented on top of public APIs [1].
Although most of applications should not try to do that, for certain
applications such as web browsers and WebView it may make sense to let
custom InputConnection implementation run with a custom Handler so that
the application can respond to the IME without blocking the UI thread.

To do that, this CL introduces a new method
InputConnection#getHandler(), which changes nothing as long as it
returns null, but if it returns non-null Handler, InputMethodManager
will use it to initialize ControlledInputConnectionWrapper.

Note that InputConnection#getHandler() is not for IME developers.
It just returns null when called in the IME process.

 [1] See Bug 24688781 for details.

Bug: 26945674
Change-Id: Id9e579bb3e2966986cdcb1c34bc8cacfeca2e1a9