ECBM SystemProperty cleanup

- Add getEmergencyCallbackMode() method in the TelephonyManager.java
to get the boolean value for ECBM callback mode from the phone through
ITelephony.aidl/java interface

- Use the added TelephonyManager Api to replace the 'get' of
PROPERTY_INECM_MODE system property in the framework/base

Change-Id: I83fbdeef5c0b43a8fc16ece044c9e191e9922bce
Test: manual
Bug: 30361624