ProcessPendingMessagesAction works based on subscriptions

PorcessPendingMessagesAction queues one message for sending/downloading
associated with a subscription triggering current action at a time. And
the ConnectivityUtil also works based on subscriptions so that pending
messages can be processed regardless of other phones' state in multi-sim
case.

It includes cleanup code as well.

Test: Manual

Change-Id: Id6b4f4a0aa6a3291e7a4d8a5d3f0fbb9db3c5b86
Signed-off-by: Taesu Lee <taesu82.lee@samsung.com>

tests/src/com/android/messaging/FakeFactory.java

@@ -46,6 +46,7 @@ import org.mockito.Mockito;
 import org.mockito.invocation.InvocationOnMock;
 import org.mockito.stubbing.Answer;
 
+import java.util.ArrayList;
 import java.util.List;
 
 public class FakeFactory extends Factory {
@@ -121,7 +122,8 @@ public class FakeFactory extends Factory {
 
                         @Override
                         public List<SubscriptionInfo> getActiveSubscriptionInfoList() {
-                            return null;
+                            // Return empty list
+                            return new ArrayList<>();
                         }
 
                         @Override