Add debug log for Lockscreen notifySmartspaceEvent.

Example log, for tapping the next alarm on Lockscreen:
```
07-02 10:13:46.851 15556 15556 D LockscreenSmartspaceController: notifySmartspaceEvent: SmartspaceTargetEvent{mSmartspaceTarget=null, mSmartspaceActionId='nextAlarmId', mEventType=1}
```

Flag: EXEMPT add debug log only
Bug: 424292340
Test: Tap next alarm, observe debug log via `adb logcat`
Change-Id: I98adc3240b362494f31b940bd7b510f622ece9fc