Add XMLRCFeedFormatter

Endpoints like XMPP natively support sending XML,
which can be advantageous in some cases.

Since most of the logic is the same as the existing
JSONRCFeedFormatter, it was moved into an abstract
MachineReadableRCFeedFormatter class, which simply
creates the array and lets the implementations format
it.

Change-Id: I44f22777cb55fc1ca6527b79ef40b340e2afbd58
Merge "Add XMLRCFeedFormatter"