[Telephony settings] Settings crashes when rotate screen in Preferred network type

When activity is recreated, fragments are automatically reattached.
Need to check the savedInstanceState is null or not when we want to add framgment in MobileNetworkActivity.
(lost to check in ag/9667137)

Bug: 154087690
Test: Manual
      make RunSettingsRoboTests -j ROBOTEST_FILTER=MobileNetworkActivityTest
Change-Id: Id169353f6dc10306dddd823a904f0803b26a6ddc
Merged-In: Id169353f6dc10306dddd823a904f0803b26a6ddc