Add javadoc about lifecycles for xml defined Controllers.

The described wiring is done in DashboardFragment.

Test: n/a
Change-Id: I086620e31a106a9183d43dd946c9e9e32170a9be

src/com/android/settings/core/BasePreferenceController.java

@@ -30,6 +30,7 @@ import android.text.TextUtils;
 import android.util.Log;
 
 import androidx.annotation.Nullable;
+import androidx.lifecycle.LifecycleObserver;
 import androidx.preference.Preference;
 import androidx.preference.PreferenceScreen;
 
@@ -38,6 +39,7 @@ import com.android.settings.slices.SettingsSliceProvider;
 import com.android.settings.slices.SliceData;
 import com.android.settings.slices.Sliceable;
 import com.android.settingslib.core.AbstractPreferenceController;
+import com.android.settingslib.core.lifecycle.Lifecycle;
 import com.android.settingslib.search.SearchIndexableRaw;
 
 import java.lang.annotation.Retention;
@@ -50,6 +52,9 @@ import java.util.List;
  * Abstract class to consolidate utility between preference controllers and act as an interface
  * for Slices. The abstract classes that inherit from this class will act as the direct interfaces
  * for each type when plugging into Slices.
+ * <p>
+ * Controllers defined in xml are automatically {@link Lifecycle#addObserver(LifecycleObserver)
+ * wired up} to the settings lifecycle if they implement {@link LifecycleObserver}.
  */
 public abstract class BasePreferenceController extends AbstractPreferenceController implements
         Sliceable {