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/layout/toolbar_search_layout.xml

@@ -44,7 +44,7 @@
             android:layout_gravity="center"
             android:contentDescription="@string/search"
             android:scaleType="fitCenter"
-            android:src="?attr/close"
+            android:src="?attr/ic_close"
             tools:ignore="RtlHardcoded"/>
     </FrameLayout>