Add InputConnection#insertContent().

Providing an official protocol for IMEs to insert an image to the
application is something that has been requested from many IME
developers to Android OS.  With this CL, IMEs are able to ask
applications to insert a content including image files as follows.

 1. An application that opts in to this protocol specifies a list of
    supported content MIME types in EditorInfo#contentMimeTypes.
 2. When an IME is actively interacting with such an application, the
    IME can call InputConnection#insertContent() with a InputContentInfo
    that contains content URI, metadata (ClipDescription), and an
    optional link URI.
 3. The application can read the stream data from the given content URI
    to insert the content into somewhere in the application.

Detailed design background can be found in the JavaDoc of
InputConnection#insertContent().

Bug: 22830793
Change-Id: Iaadf934a997ffcd6000a516cc3c1873db56e60ad