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