Added command line tool to enable/disable new data stack

Provide the temporary command line tool to check whether new
data stack is enabled or not.

Bug: 196597630
Test: adb shell cmd phone get-data-mode
Change-Id: I9f95e7424e6e74af12a0f6780a58fb5732df7300

telephony/java/com/android/internal/telephony/ITelephony.aidl

@@ -2502,4 +2502,7 @@ interface ITelephony {
      */
     CellIdentity getLastKnownCellIdentity(int subId, String callingPackage,
             String callingFeatureId);
+
+    /** Check if telephony new data stack is enabled. */
+    boolean isUsingNewDataStack();
 }