Display notification filter/clear actions as menu items (#3877)

Previously the notification filter and clear actions were shown as
buttons in the UI, with a preference that determined whether they were
displayed.

Remove this preference, and display them as menu items.

- "Filter notifications" is shown as an icon, if possible
- "Clear notifications" is only ever shown as a menu item, to reduce the
chance the user inadvertently selects it

To ensure that the options menu appears correctly, remove the code that
creates a "fake" action bar, and adjust the layouts so that there are
three toolbars;

- mainToolbar -- displays the icons, and the current "location" (Home,
Notifications, etc)
- topNav -- displays the row of tabs at the top
- bottomNav -- displays the row of tabs at the bottom

Only one of them is set as the support action bar (depending on the
user's preferences). This provides the "show a logo" and "show the
options menu" functionality as standard, without needing to re-implement
as the previous code did.

app/src/main/res/values/styles.xml

@@ -179,9 +179,4 @@
         <item name="materialDrawerMaskDrawable">@drawable/materialdrawer_shape_small</item>
         <item name="materialDrawerDrawCircularShadow">false</item>
     </style>
-
-    <style name="ShapeAppearance.Avatar" parent="ShapeAppearance.MaterialComponents">
-        <item name="cornerFamily">rounded</item>
-        <item name="cornerSize">12.5%</item>
-    </style>
 </resources>