SipService: handle connectivity changes correctly.

This patch assumes that for the same network type,
there MUST be a DISCONNECTED between two CONNECTEDs.
Also removes the Wi-Fi scanning since the framework
already handles this when a WifiLock is held.

Bug: 4283795
Change-Id: I08481e70c651cffcbb516c8cc6584c919078fa4f

Author: Chia-chi Yeh