Tell the reason why IMMS unbinds IME client.

As a preparation to fix Bug 25373872, this introduce an additional
parameter to IInputMethodClient.unbind() so that the IME client
that is running in the application side can know why IMMS needs
to unbind the connection.

In future the "unbindReason" parameter is supposed to be used to
optimize the behavior of IMM runtime running in the application process,
but for now it is not used unless we build the sytem with debug message
enabled.  Hence there should be no user-visible change with this CL.

Bug: 25373872
Change-Id: I74e83c8ca9d1d53e31e9c7b5bda1dec6274e59c8