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/drawable/ic_repeat_white_24dp.xml

@@ -0,0 +1,5 @@
+<vector android:height="24dp" android:tint="#FFFFFF"
+    android:viewportHeight="24.0" android:viewportWidth="24.0"
+    android:width="24dp" xmlns:android="http://schemas.android.com/apk/res/android">
+    <path android:fillColor="#FF000000" android:pathData="M7,7h10v3l4,-4 -4,-4v3L5,5v6h2L7,7zM17,17L7,17v-3l-4,4 4,4v-3h12v-6h-2v4z"/>
+</vector>