Add subscription specific API for IMS registration status

Add a new API "isImsRegisteredForSubscriber(int subId)" so that IMS
registration status for specific subscription is acquired.

Bug: 28806101
Test: Manually Enable/Disable IMS and view Settings
Merged-In: I6c452c512f03cf41704b91331e44141ed3050cf9
Change-Id: I6c452c512f03cf41704b91331e44141ed3050cf9
diff --git a/src/com/android/phone/PhoneInterfaceManager.java b/src/com/android/phone/PhoneInterfaceManager.java
index c06e34e..7493858 100644
--- a/src/com/android/phone/PhoneInterfaceManager.java
+++ b/src/com/android/phone/PhoneInterfaceManager.java
@@ -3276,6 +3276,21 @@
         return mPhone.isImsRegistered();
     }
 
+    /**
+     * {@hide}
+     * Returns the IMS Registration Status on a particular subid
+     *
+     * @param subId
+     */
+    public boolean isImsRegisteredForSubscriber(int subId) {
+        Phone phone = getPhone(subId);
+        if (phone != null) {
+            return phone.isImsRegistered();
+        } else {
+            return false;
+        }
+    }
+
     @Override
     public int getSubIdForPhoneAccount(PhoneAccount phoneAccount) {
         return PhoneUtils.getSubIdForPhoneAccount(phoneAccount);