Extend Intent/Uri conversion for use by Browser

This introduces a new Uri form of Intent with an "intent:" scheme, and a
corresponding update to the parser to handle these, so that the browser
can use this generic facility for starting activities based on the links
that are clicked and allow for web pages to link to arbitrary intents.

There is also a new "package" field on Intent which allows you to limit
the components it finds to a given package.  This replaces the new method
that was added to PackageManger for doing this when resolving activities,
and implements it for all Intent queries against the package manager.