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).

Bug: 210774375
Test: atest ConnectivityServiceTest
Change-Id: I2c7b291fee3450f588a282f7a252c183c374becf
(cherry picked from commit 1e6438b43a13c27d0f939ec417dc0a84597fe935)