Fixed unmetered network request caused data leak

For some carriers, MMS (unmetered) and internet (metered)
is sharing the same APN. When the user sends MMS
when data is turned off, internet will become available
after sending the MMS. That is because connectivity
service sees there are lots of internet requests
that can be satisfied by this network, so the tear
down request will not be sent from connectivity service
after MMS is sent.

When unmetered request brings up the network that
also supports metered usage, we should remove those
metered capabilities. This can make sure this network
can be only used for unmetered purposes.

Also fixed a bug that MMS-always-allowed feature should
be only applicable to data disabled case, not roaming
disabled case.

Fix: 220171983
Test: Manual & atest DataNetworkControllerTest
Change-Id: I30d781c45b7a1c89646ec6d168928e5289dc5575