CSSMin: factor out image encoding into encodeImageAsDataURI method

The code for encoding suitable images as base64 data URIs was previously part
of the remap method of CSSMin, which greps CSS files for /* @embed */
annotations. This patch moves it into its own method, encodeImageAsDataURI.

Change-Id: I6f57116c6a6c18ff9d4e28760dd484ea2c8fc493