Do not dynamically change MMTEL capability

This CL partially reverted ag/17922489, which dynamically changes
MMTEL capability. If the network request contains MMTEL, but telephony
remove it, connectivity service will consider the request can't be
satisfied anymore. So it will initiate tear down. So we should not
remove MMTEL once it's reported as the network's capability. So the
behavior becomes

1. If the network request contains only IMS, but not MMTEL, moving
   between VoPS and non-VoPS will not cause any IMS PDN tear down.
2. If the network request contains both IMS and MMTEL, when the
   device moves from VoPS to non-VoPS, telephony will
   1. When voice call is ongoing, telephony will NOT tear down the
      IMS PDN if delay IMS tear down carrier config is turned on.
   2. When voice call is not ongoing, telephony will initiate
      graceful IMS tear down, and let IMS service has chance to
      perform IMS de-registration.

Fix: 233912777
Test: Basic testing + atest DataNetworkControllerTest DataNetworkTest
Change-Id: Ie51c319ea26fd54f1a0846e1c3cb228481a67230