Add storage dashborad page.

Refactor shared dashboard page display logic between storage and
system page.

Bug: 31800690
Test: RunSettingsRoboTests

Change-Id: I7eb86f590b79ab871bdb383fb2f5326790beb193

src/com/android/settings/Settings.java

@@ -164,6 +164,7 @@ public class Settings extends SettingsActivity {
     public static class SystemSettings extends SettingsActivity { /* empty */ }
 
     // Top level categories for new IA
+    public static class StorageDashboardActivity extends SettingsActivity {}
     public static class SystemDashboardActivity extends SettingsActivity {}
     public static class SupportDashboardActivity extends SettingsActivity {}