[Material Next] Support "Learn more" for help link

Allow CaptionManager, Magnification, Color correction, Color inversion,
Time to take action, and Autoclick.

Bug: 187671696
Test: make RunSettingsRoboTests -j52 ROBOTEST_FILTER=AccessibilityFooterPreferenceTest
Change-Id: I9d753243d4f61b6b3d58b50425b7a75a812a481d

src/com/android/settings/accessibility/ToggleScreenMagnificationPreferenceFragment.java

@@ -296,6 +296,11 @@ public class ToggleScreenMagnificationPreferenceFragment extends
                 getShortcutTypeSummary(getPrefContext()));
     }
 
+    @Override
+    public int getHelpResource() {
+        return R.string.help_url_magnification;
+    }
+
     @Override
     public int getMetricsCategory() {
         // TODO: Distinguish between magnification modes