Added a new network capability MMTEL

Added NET_CAPABILITY_MMTEL to indicate a network
support MMTEL (Multimedia telephony). This is for
IMS service to request an IMS network that supports
voice or PS (VoPS for 4G, VoNR for 5G).

Fix: 210774375
Test: atest ConnectivityServiceTest
Change-Id: I633a681c1c34ce6d28929c3e6acf16c711588adf