[Settings] Change the restart policy for MobileNetworkActivity

Prevent recreating the fragment in MobileNetworkActivity when the
subId is the same.

Bug: 141833767
Test: m RunSettingsRoboTests -j ROBOTEST_FILTER=MobileNetworkActivityTest
Change-Id: I8167781fb462efa310a5d6bac76d880dbc9b6bfc
1 file changed