Add direct support for HTML formatted text in ClipData etc.

When using the clipboard, ACTION_SEND, etc., you can now supply
HTML formatted text as one of the representations.  This is exposed
as a set of methods on ClipData for building items with HTML
formatted text, and retrieving and coercing to HTML (and styled)
text.  In addtion, there is a new EXTRA_HTML_TEXT for interoperating
with the old ACTION_SEND protocol.

Change-Id: I8846520a480c8a5f829ec1e693aeebd425ac170d