ContentCapture: Notify when Composing region changes.

Sending the notification from the InputConnection layer reduces the
number of events that are handled (merged) by ContentCapture. We are
also able to handle composing span being removed; at the TextView layer,
we cannot do that as we don't know if the composing span will be added
back, which happens on typing).

Bug: 184311217
Test: manual - append chars, delete chars, replace text, move cursor to
 composing text, move cursor to remove composing span, drag cursor
Test: atest android.contentcaptureservice.cts.LoginActivityTest
Change-Id: I0e9d153cf7ba2734f38cb1b6044eb7b670fb34ad