Allow to add/remove background color to/from the committed text

This is a groundwork for subsequent CLs where we need to
add/remove background color to/from the commited text.

In this CL, we use Spanned#SPAN_COMPOSING so that we can easily
remove such a background color by calling
InputConnection#finishComposingText. To make this operation easy
and realiable, we need to track whether we have specified the
background color to the commited text or not at one place. Here
we use RichInputConnection for this purpose.

Change-Id: I5f9bc4425c5d1b80a719a20e5baf336729ec08d2