Make TelephonyManager#getEmergencyCallbackMode SystemApi

BUG: 67750821
Test: android.telephony.cts.TelephonyManagerTest#testTelephonyManager().
Basic test to ensure that the method does not throw any exception.

Change-Id: I00f0e62803810b2650eba7562a84774369e917c6

telephony/java/android/telephony/TelephonyManager.java

@@ -6866,6 +6866,8 @@ public class TelephonyManager {
      * @return true if phone is in emergency callback mode
      * @hide
      */
+    @SystemApi
+    @RequiresPermission(android.Manifest.permission.READ_PRIVILEGED_PHONE_STATE)
     public boolean getEmergencyCallbackMode() {
         return getEmergencyCallbackMode(getSubId());
     }