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/player_popup_notification.xml

@@ -1,6 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <LinearLayout
     xmlns:android="http://schemas.android.com/apk/res/android"
+    xmlns:app="http://schemas.android.com/apk/res-auto"
     xmlns:tools="http://schemas.android.com/tools"
     android:id="@+id/notificationContent"
     android:layout_width="match_parent"
@@ -58,7 +59,7 @@
         android:focusable="true"
         android:padding="5dp"
         android:scaleType="fitCenter"
-        android:src="@drawable/ic_repeat_white"
+        app:srcCompat="@drawable/ic_repeat_white_24dp"
         tools:ignore="ContentDescription"/>
 
     <ImageButton
@@ -68,7 +69,7 @@
         android:background="#00000000"
         android:clickable="true"
         android:focusable="true"
-        android:src="@drawable/ic_pause_white"
+        app:srcCompat="@drawable/ic_pause_white_24dp"
         tools:ignore="ContentDescription"/>
 
     <ImageButton