ECBM SystemProperty cleanup

- Add getEmergencyCallbackMode() method to get the boolean
value for ECBM callback mode from the phone, in the
PhoneInterfaceManager.java

- Replace the 'get' of PROPERTY_INECM_MODE system property
in the packages/services/Telephony

Change-Id: Ide35e28b3339f58523bfe92be3c2fdfce111d45e
Test: Manual
Bug: 30361624
diff --git a/src/com/android/phone/PhoneInterfaceManager.java b/src/com/android/phone/PhoneInterfaceManager.java
index 52c2a79..3f61b34 100644
--- a/src/com/android/phone/PhoneInterfaceManager.java
+++ b/src/com/android/phone/PhoneInterfaceManager.java
@@ -3807,4 +3807,18 @@
             phone.setSimPowerState(powerUp);
         }
     }
+
+    /**
+     * Check if phone is in emergency callback mode
+     * @return true if phone is in emergency callback mode
+     * @param subId sub id
+     */
+    public boolean getEmergencyCallbackMode(int subId) {
+        final Phone phone = getPhone(subId);
+        if (phone != null) {
+            return phone.isInEcm();
+        } else {
+            return false;
+        }
+    }
 }