Get rid of extra whitespace at top of Network & internet page

In DSDS mode, we put an extra header at the top of the Network &
internet page with a listing of your available connectivity (mobile
subscriptions, plus wifi). The PreferenceCategory that contains these
prefs has space reserved for a title by default, but we don't need or
want a title here. This CL gets rid of that extra space by using the
custom layout we already have and use in other places for this purpose.

Fixes: 129562555
Test: visual (get device into DSDS mode with multiple SIMs, and look at
the header at the top of the Network & internet page - there should be
no extra space at the top)

Change-Id: I8f9aae9c07303b2d7ecfc8b6722a4d66555392f1

res/xml/network_and_internet_v2.xml

@@ -23,6 +23,7 @@
     <PreferenceCategory
         android:key="multi_network_header"
         android:title="@string/summary_placeholder"
+        android:layout="@layout/preference_category_no_label"
         settings:allowDividerBelow="true"
         android:order="-40"
         settings:controller="com.android.settings.network.MultiNetworkHeaderController"/>