Use vector drawables instead of PNGs for material icons

For all manually-created images PNG have been kept.
- rename all icon attrs to have a `ic_` prefix
- always use `_24dp` icons, because there is no real difference, since they are vector drawables
- always use the original name found on material.io for icon drawables, as to not create confusion and possibly duplicates. Icon names can still be different from real drawable names, though I have made some of them compliant to this or maybe more meaningul.
- remove duplicate `getIconByAttr()` in ThemeHelper (use `resolveResourceIdFromAttr()`
- use standard icons for `expand_more` and `expand_less` instead of triangles
- use `play_button_outline` instead of custom PNG as play button in VideoDetailFragment (questionable, as there is no shadow anymore)

app/src/main/res/menu/video_detail_menu.xml

@@ -4,13 +4,13 @@
 
     <item
         android:id="@+id/action_play_with_kodi"
-        android:icon="?attr/cast"
+        android:icon="?attr/ic_cast"
         android:title="@string/play_with_kodi_title"
         app:showAsAction="ifRoom"/>
 
     <item
         android:id="@+id/menu_item_share"
-        android:icon="?attr/share"
+        android:icon="?attr/ic_share"
         android:title="@string/share"
         app:showAsAction="ifRoom"/>