SIP: misc fixes.

+ Fix keepalive timer event leak due to the race between stopping timer and
  the async'ed timeout handler
+ SipSessionImpl: set state before handling an event to ensure we get correct
  state when some error occurs during handling the event.
+ Fix potential NPE in SipManager.ListenerRelay.getUri().

Change-Id: I021ee34f83059fd4fbb64b30bea427a5462aa51b