Add 12dp padding above the first slice item

-Add an extra padding in adapter for dedicated panels
-Add test cases

Bug: 154181365
Test: manual test
make -j50 RunSettingsRoboTests
"adb shell am start -a android.settings.panel.action.INTERNET_CONNECTIVITY"

Change-Id: I65bc26c23ca5a7f1b433c36ac7ddf1a658793719

res/layout/panel_slice_slider_row.xml

@@ -16,6 +16,7 @@
 -->
 <LinearLayout
     xmlns:android="http://schemas.android.com/apk/res/android"
+    android:id="@+id/slice_slider_layout"
     android:layout_width="match_parent"
     android:layout_height="wrap_content"
     android:orientation="vertical">