Add getSubscriptionId API

The old getSubId or getSubscriptionIds return multiple sub ids
per slot index, which is not possible in today's implementation
because the slot index here refers to the logical slot index,
which is also known as phone id, or the logical modem index. In
today's telephony, one logical phone can only have one
subscription at one time, so does the modem.

Bug: 239607619
Test: Manual
Change-Id: I0a43d33daf523e6f0bea054c8965281bafbbcaa4

src/com/android/settings/sim/smartForwarding/DisableSmartForwardingTask.java

@@ -57,10 +57,6 @@ public class DisableSmartForwardingTask implements Runnable {
     }
 
     private int getSubId(int slotIndex) {
-        int[] subId = SubscriptionManager.getSubId(slotIndex);
-        if (subId != null && subId.length > 0) {
-            return subId[0];
-        }
-        return SubscriptionManager.INVALID_SUBSCRIPTION_ID;
+        return SubscriptionManager.getSubscriptionId(slotIndex);
     }
 }