Adopt HTML/CSS color values in fromHtml()

- "green", "gray/grey", "lightgray/lightgrey", and
  "darkgray/darkgrey" have different values in HTML/CSS.

- Adopt HTML/CSS color values according to user specified
  option flag.

- Remove support to "@color" notation as it is not standard
  HTML/CSS.

- Replace SPAN_MARK_MARK with SPAN_INCLUSIVE_EXCLUSIVE
  consistency.

BUG:26950749
Change-Id: I5b33282368042f74f1b4768e096aae3e5bf45de6