If your goal is that you want NightClock to be started whenever an ACTION_POWER_CONNECTED broadcast is sent, your approach of using a BroadcastReceiver is fine. However, do not register it from an activity. Rather, register it in the manifest:
<receiver android:name=".OnPowerReceiver">
<intent-filter>
<action android:name="android.intent.action.ACTION_POWER_CONNECTED" />
</intent-filter>
</receiver>
Then, have your BroadcastReceiver as a public Java class (here named OnPowerReceiver, though you can call it whatever you want), and have it call startActivity().
Bear in mind that users probably do not want you doing this. There are many other cases for connecting a phone to power besides starting a “night clock”. I humbly suggest you simply let users start your activity via the home screen.