inputmethod: Make sure we send a state commit

Detach from the serials that the client send us, since they don't really
matter to the input method and track our own.
Also issue a commit state whenever we adopt a new input context. This
can happen for reasons that don't come from the client, like for example
the input method has just been enabled.
In practice, this solves an issue that we can easily see with maliit
where it would be left in a half-initialised state and would show
autocorrection fields on terminal applications when unnecessary and
possibly similar problems every now and then.

(cherry picked from commit 184e53e5)