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/play_queue_item.xml

@@ -6,7 +6,7 @@
     android:layout_width="match_parent"
     android:layout_height="48dp"
     android:foreground="?attr/selectableItemBackground"
-    android:background="?attr/selector_drawable"
+    android:background="?attr/selector"
     android:clickable="true"
     android:focusable="true"
     android:paddingTop="6dp"
@@ -22,7 +22,7 @@
         android:layout_marginLeft="2dp"
         android:layout_marginRight="2dp"
         android:scaleType="fitXY"
-        android:src="?attr/selected"
+        android:src="?attr/ic_selected"
         tools:ignore="ContentDescription,RtlHardcoded"/>
 
     <ImageView
@@ -46,7 +46,7 @@
         android:layout_alignParentRight="true"
         android:padding="@dimen/video_item_search_image_right_margin"
         android:scaleType="center"
-        android:src="?attr/drag_handle"
+        android:src="?attr/ic_drag_handle"
         tools:ignore="ContentDescription,RtlHardcoded"/>
 
     <TextView