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/EmergencyCallbackModeExitDialog.java b/src/com/android/phone/EmergencyCallbackModeExitDialog.java
index b423e14..f5509b9 100644
--- a/src/com/android/phone/EmergencyCallbackModeExitDialog.java
+++ b/src/com/android/phone/EmergencyCallbackModeExitDialog.java
@@ -80,8 +80,7 @@
 
         mPhone = PhoneGlobals.getInstance().getPhoneInEcm();
         // Check if phone is in Emergency Callback Mode. If not, exit.
-        final boolean isInEcm = Boolean.parseBoolean(
-                SystemProperties.get(TelephonyProperties.PROPERTY_INECM_MODE));
+        final boolean isInEcm = mPhone.isInEcm();
         Log.i(TAG, "ECMModeExitDialog launched - isInEcm: " + isInEcm + " phone:" + mPhone);
         if (mPhone == null || !isInEcm) {
             finish();