Add Null Cipher Feature Flag Name to TelephonyManager

Bug: 262063758
Test: atest GsmCdmaPhoneTest
Test: atest NullAlgorithmsPreferenceControllerTest
Test: adb shell device_config put cellular_security
enable_null_cipher_toggle_true; // On cuttlefish this will get you a
settings toggle

Change-Id: If97f40f1e48e82ece71133c61d01b1e89be5173f

telephony/java/android/telephony/TelephonyManager.java

@@ -388,6 +388,9 @@ public class TelephonyManager {
     /** @hide */
     public static final int INVALID_PORT_INDEX = -1;
 
+    /** @hide */
+    public static final String PROPERTY_ENABLE_NULL_CIPHER_TOGGLE = "enable_null_cipher_toggle";
+
     private final Context mContext;
     private final int mSubId;
     @UnsupportedAppUsage