Switch ExpandablePreferenceGroup to use SL theme

ExpandablePreferenceGroup wasn't picking up the default theme because
the default style was being explicitly set to the system preferenceStyle
rather than the Support Library style.

This is a code simplification only. It has no effect on the UI. Verified
on KitKat, Lollipop and Pie.

Bug: 985727
Change-Id: I7d56be2c0b8c4ef4aed3c30caede4999a47bd604
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1776727Reviewed-by: Boris Sazonov <bsazonov@chromium.org>
Commit-Queue: Natalie Chouinard <chouinard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#692100}

chrome/android/java/src/org/chromium/chrome/browser/preferences/ExpandablePreferenceGroup.java

@@ -28,11 +28,7 @@ public class ExpandablePreferenceGroup extends PreferenceGroup {
     private Drawable mDrawable;
 
     public ExpandablePreferenceGroup(Context context, AttributeSet attrs) {
-        super(context, attrs, android.R.attr.preferenceStyle);
-
-        // TODO(crbug.com/985727): Determine why PreferenceGroup doesn't automatically use the
-        // preference layout set by the theme.
-        setLayoutResource(R.layout.preference_compat);
+        super(context, attrs, R.attr.preferenceStyle);
 
         setWidgetLayoutResource(R.layout.checkable_image_view_widget);
     }