Add new column user_editable to Carriers table in telephony db.

Test: Verified manually that an APN with user_editable=false cannot be
modified via UI
Bug: 62852457
Change-Id: I67f1c5c537978d54aeea0a726d4056593381aa2d

telephony/java/android/telephony/Telephony.java

@@ -2764,6 +2764,13 @@ public final class Telephony {
          */
         public static final String USER_VISIBLE = "user_visible";
 
+        /**
+         * Is the user allowed to edit this APN?
+         * <p>Type: INTEGER (boolean) </p>
+         * @hide
+         */
+        public static final String USER_EDITABLE = "user_editable";
+
         /**
          * Following are possible values for the EDITED field
          * @hide