New Linkify.addLinks function with set of known schemes

A new addLinks function is added that accepts a default scheme and a
set of known schemes. Default scheme is applied whenever the link found
does not start with one of the given known schemes.

Moreover, previously JavaDoc for addLinks functions with a single scheme
parameter described that the scheme attribute will be prepended to the
links that do not have 'a' scheme. The code actually checks if the link
starts with the given scheme and prepends if not. JavaDoc updated
accordingly.

Bug: 26985901
Change-Id: I94ea81dcf83ba7a6b6cd47c10fe8fb277964eb15