[ExpressiveBattery] Update BatteryHeaderTextPreference

- Remove background container
- Update the distance between status text and progress bar from 16dp to 10dp.

Bug: 349652542
Test: visual
Flag: EXEMPT flag by System prop
Change-Id: I1cd6f597e01daad9fbc5fb128926c8c153ff0328

res/layout/preference_battery_header_text.xml

@@ -18,8 +18,9 @@
     android:layout_width="match_parent"
     android:layout_height="wrap_content"
     android:orientation="vertical"
-    android:layout_marginStart="?android:attr/listPreferredItemPaddingStart"
-    android:layout_marginEnd="?android:attr/listPreferredItemPaddingEnd"
+    android:paddingStart="?android:attr/listPreferredItemPaddingStart"
+    android:paddingEnd="?android:attr/listPreferredItemPaddingEnd"
+    android:layout_marginTop="-6dp"
     android:paddingBottom="16dp">
 
     <TextView

src/com/android/settings/fuelgauge/BatteryHeaderTextPreference.java

@@ -26,9 +26,10 @@ import androidx.preference.Preference;
 import androidx.preference.PreferenceViewHolder;
 
 import com.android.settings.R;
+import com.android.settingslib.widget.GroupSectionDividerMixin;
 
 /** A preference for battery header text. */
-public class BatteryHeaderTextPreference extends Preference {
+public class BatteryHeaderTextPreference extends Preference implements GroupSectionDividerMixin {
     private static final String TAG = "BatteryHeaderTextPreference";
 
     @Nullable private CharSequence mText;