Add Testing Hook For TelephonyManager ISub Member

New hook allows tests to set a mock for the SubscriptionManagerService

Bug: 266472206
Test: atest GsmCdmaPhoneTest # see topic
Change-Id: I585af448847a9bb73dc3e22757874d28e54bc7cc

telephony/java/android/telephony/TelephonyManager.java

@@ -16355,6 +16355,7 @@ public class TelephonyManager {
 
     /**
      * Setup sIPhoneSubInfo for testing.
+     *
      * @hide
      */
     @VisibleForTesting
@@ -16365,9 +16366,21 @@ public class TelephonyManager {
     }
 
     /**
-     * Whether device can connect to 5G network when two SIMs are active.
+     * Setup sISub for testing.
+     *
      * @hide
-     * TODO b/153669716: remove or make system API.
+     */
+    @VisibleForTesting
+    public static void setupISubForTest(ISub iSub) {
+        synchronized (sCacheLock) {
+            sISub = iSub;
+        }
+    }
+
+    /**
+     * Whether device can connect to 5G network when two SIMs are active.
+     *
+     * @hide TODO b/153669716: remove or make system API.
      */
     public boolean canConnectTo5GInDsdsMode() {
         ITelephony telephony = getITelephony();