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_stars_white_24dp.xml

@@ -1,9 +1,5 @@
-<vector xmlns:android="http://schemas.android.com/apk/res/android"
-        android:width="24dp"
-        android:height="24dp"
-        android:viewportHeight="24.0"
-        android:viewportWidth="24.0">
-    <path
-        android:fillColor="#FFFFFFFF"
-        android:pathData="M11.99,2C6.47,2 2,6.48 2,12s4.47,10 9.99,10C17.52,22 22,17.52 22,12S17.52,2 11.99,2zM16.23,18L12,15.45 7.77,18l1.12,-4.81 -3.73,-3.23 4.92,-0.42L12,5l1.92,4.53 4.92,0.42 -3.73,3.23L16.23,18z"/>
+<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="M11.99,2C6.47,2 2,6.48 2,12s4.47,10 9.99,10C17.52,22 22,17.52 22,12S17.52,2 11.99,2zM16.23,18L12,15.45 7.77,18l1.12,-4.81 -3.73,-3.23 4.92,-0.42L12,5l1.92,4.53 4.92,0.42 -3.73,3.23L16.23,18z"/>
 </vector>