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_computer_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="M21,2L3,2c-1.1,0 -2,0.9 -2,2v12c0,1.1 0.9,2 2,2h7v2L8,20v2h8v-2h-2v-2h7c1.1,0 2,-0.9 2,-2L23,4c0,-1.1 -0.9,-2 -2,-2zM21,16L3,16L3,4h18v12z"/>
+<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="M20,18c1.1,0 1.99,-0.9 1.99,-2L22,6c0,-1.1 -0.9,-2 -2,-2H4c-1.1,0 -2,0.9 -2,2v10c0,1.1 0.9,2 2,2H0v2h24v-2h-4zM4,6h16v10H4V6z"/>
 </vector>