SystemUI: Fix issue that log can't be enabled in MobileSignalController

mTag in MobileSignalController contain "(" and ")", which can't be
analyzed by Log.isLoggable,so use DEBUG to control the logs.

Bug: 228813927
Test:adb root & adb remount & adb shell "echo 'log.tag.NetworkController=V' >> /system/build.prop" & adb reboot
     adb logcat -v time *:S NetworkController.MobileSignalController(1) NetworkController.MobileSignalController(2)
Change-Id: I97992455c539665b507a4035eea5941d0081e8a9

packages/SystemUI/src/com/android/systemui/statusbar/connectivity/MobileSignalController.java

@@ -110,7 +110,7 @@ public class MobileSignalController extends SignalController<MobileState, Mobile
                 @Override
                 public void onMobileStatusChanged(boolean updateTelephony,
                         MobileStatus mobileStatus) {
-                    if (Log.isLoggable(mTag, Log.DEBUG)) {
+                    if (DEBUG) {
                         Log.d(mTag, "onMobileStatusChanged="
                                 + " updateTelephony=" + updateTelephony
                                 + " mobileStatus=" + mobileStatus.toString());
@@ -719,7 +719,7 @@ public class MobileSignalController extends SignalController<MobileState, Mobile
      * This will call listeners if necessary.
      */
     private void updateTelephony() {
-        if (Log.isLoggable(mTag, Log.DEBUG)) {
+        if (DEBUG) {
             Log.d(mTag, "updateTelephonySignalStrength: hasService="
                     + mCurrentState.isInService()
                     + " ss=" + mCurrentState.signalStrength