Add callbacks to notify View when an InputConnection is opened/closed

Added hidden callback methods on View to notify editors when the
system has initialized an InputConnection successfully and when
the InputConnection is closed.

This allows editable views (e.g. EditText) to have access to the
final EditorInfo and InputConnection that were created via
onCreateInputConnection(). This enables the editor platform code
to do things such as
* know what type of content can be inserted into the view (ie, the
MIME types in EditorInfo.contentMimeTypes)
* call APIs on InputConnection, e.g. commitContent() to pass
non-text content to the application

Bug: 163400105
Test: Manual and unit test
  atest FrameworksCoreTests:ViewInputConnectionTest
Change-Id: Iaafb0a03126c9292c24415f866dbdd72cadfa239