Add watermark support.

Use these system properties:

ro.watermark.text
ro.watermark.height
ro.watermark.x
ro.watermark.y
ro.watermark.color
ro.watermark.shadow.radius
ro.watermark.shadow.dx
ro.watermark.shadow.dy
ro.watermark.shadow.color

If ro.watermark.text is not set, no watermark will be shown.  All others
have reasonable defaults if they are not set.

Change-Id: Ibe4a01e6f1c576494ae2462e2688cdfaa8c62cb8