[Android] Add vertical scrollbars on Settings

Show vertical scroll bars on settings screens, matching the Material
style. These scroll bars appear briefly when the settings screen is
opened, and again whenever the user scrolls the page to show their
current scroll position.

This change only affects settings screens that extend
PreferenceFragmentCompat. Other custom setting screens will be addressed
in follow-up bugs (i.e. https://crbug.com/998609).

Bug: 1131073
Change-Id: Ie9c9a7f118e9a00d5f5439d285fb67958b83b1d3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2424506
Auto-Submit: Natalie Chouinard <chouinard@chromium.org>
Reviewed-by: Wenyu Fu <wenyufu@chromium.org>
Commit-Queue: Natalie Chouinard <chouinard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810022}

components/browser_ui/settings/android/java/res/values/styles.xml

@@ -31,6 +31,7 @@
     </style>
 
     <style name="PreferenceTheme">
+        <item name="android:scrollbars">vertical</item>
         <item name="preferenceStyle">@style/PreferenceItem</item>
         <item name="preferenceFragmentCompatStyle">@style/SettingsFragment</item>
         <item name="preferenceFragmentListStyle">@style/SettingsFragmentList</item>