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-land/activity_player_queue_control.xml

@@ -139,7 +139,7 @@
                 android:focusable="true"
                 android:scaleType="fitCenter"
                 android:tint="?attr/colorAccent"
-                android:src="@drawable/ic_pause_white"
+                app:srcCompat="@drawable/ic_pause_white_24dp"
                 tools:ignore="ContentDescription"/>
 
             <ProgressBar
@@ -213,7 +213,7 @@
                 android:focusable="true"
                 android:scaleType="fitXY"
                 android:tint="?attr/colorAccent"
-                android:src="@drawable/ic_repeat_white"
+                app:srcCompat="@drawable/ic_repeat_white_24dp"
                 tools:ignore="ContentDescription"/>
 
             <View android:layout_width="10dp"
@@ -233,7 +233,7 @@
                 android:focusable="true"
                 android:scaleType="fitXY"
                 android:tint="?attr/colorAccent"
-                android:src="@drawable/ic_shuffle_white_24dp"
+                app:srcCompat="@drawable/ic_shuffle_white_24dp"
                 tools:ignore="ContentDescription"/>
 
             <TextView
@@ -262,7 +262,7 @@
         android:gravity="center"
         android:orientation="horizontal"
         android:paddingLeft="16dp"
-        android:background="@drawable/player_controls_bg"
+        android:background="@drawable/player_controls_background"
         android:paddingRight="16dp">
 
         <TextView