Android treats the BusyStatus as outlook does

Summary:
When accepting an invitation over android and the event is being added
subsequently to the calendar from the server, the client seems to rely
on the busy status to set our attendance status.
Similarly, if an event is accepted from the android calendar it does not
send an attendee with an attendeestatus, but a busystatus set.

Differential Revision: https://git.kolab.org/D4532