Calls API: allow to reject call as "contact is busy"

[adbenitez]

when user is already in a call, and a new call arrives it is common to auto-reject the call in "contact is busy" mode, in this mode:

• the receiver calls a rpc.rejectAsBusy(accId, msgId) API similar to the existing "reject call" API but:
• the call is marked as "missed call" for the receiver/rejector (the caller gets canceled as manually canceled for now)
• the call remains un-seen as with normal missed calls

this is how Signal and other apps work (also normal phone calls, if you are in a call new callers get rejection) and it is needed for DC android implementation because current approach only supports a single call at a time using the Android APIs and if a new call is made currently it is not notified nor are any way to reject it or pick it