Add IC#closeConnection().

It turns out that BaseInputConnection has still depended on a private
API named BaseInputConnection#reportFinish(), which was introduced
4 years ago to work around a UI freeze due to an unbalanced batch edit
count [1].  Note that such an unbalanced batch edit count cannot always
be avoidable.  It can easily occur in the following situations.
 - The current IME crashed during batch edit.
 - The user changed the View focus during batch edit.
 - The current IME called IMM#switchToNextInputMethod() during batch
   edit.

The remaining problem is that #reportFinish() is still an internal API
and only subclasses of BaseInputConnection can implement it, and IMM
calls it when and only when the current InputConnection is
BaseInputConnection or its subclass.  InputConnectionWrapper and any
other InputConnection implementations will never receive such a callback
to clean up InputConnection#{begin, end}BatchEdit(), which is considered
to be a major contributor to UI freeze.

To address the above issue, we unhide BaseInputConnection#reportFinish()
as InputConnection#closeConnection() so that application developers can
receive an appropriate callback to clean up internal state including
unfinished batch edit.

  [1] I5525d776916f0c42d5e6d4a4282aed590d7f0e9a
      9d69ecbf

Bug: 24688781
Bug: 25332806
Change-Id: I234309c5880c9fe0b299b8bd0f8862796d4dda0d