Enhance support to text size spans in toHtml()

Convert AbsoluteSizeSpan to <span> with CSS font-size property with CSS
px as the unit of measurement. Sizes measured in Android px are converted
to dip, since there is no CSS unit which conveys the idea of a device
pixel.

Add support to RelativeSizeSpan, which converts to <span> with CSS
font-size property with em as the unit of measurement.

Change-Id: I9104cc5f9b00ab1a31929565dc49e0026466cc75