CallManager/SipPhone: fix reject a ringing call

+ CallManager: fix getFirstActiveRingingCall(), getActiveFgCall(), getFirstActiveBgCall()
+ Set DisconnectCause to be INCOMING_REJECTED when a call is rejected

http://b/issue?id=3049671

Change-Id: Ica1d81ca4b71ab0ceb2ab437b82bbb4ccf86fe92