Use Material Design drawables for the SimpleExoPlayer player control buttons.

library/src/main/res/drawable-v21/exo_controls_fastforward.xml

+<vector xmlns:android="http://schemas.android.com/apk/res/android"
+  android:width="32dp"
+  android:height="32dp"
+  android:viewportHeight="24.0"
+  android:viewportWidth="24.0">
+
+  <path android:fillColor="#FFF" android:pathData="M4,18l8.5,-6L4,6v12zM13,6v12l8.5,-6L13,6z"/>
+</vector>

library/src/main/res/drawable-v21/exo_controls_next.xml

+<vector xmlns:android="http://schemas.android.com/apk/res/android"
+  android:width="32dp"
+  android:height="32dp"
+  android:viewportHeight="24.0"
+  android:viewportWidth="24.0">
+
+  <path android:fillColor="#FFF" android:pathData="M6,18l8.5,-6L6,6v12zM16,6v12h2V6h-2z"/>
+</vector>

library/src/main/res/drawable-v21/exo_controls_pause.xml

+<vector xmlns:android="http://schemas.android.com/apk/res/android"
+  android:width="32dp"
+  android:height="32dp"
+  android:viewportHeight="24.0"
+  android:viewportWidth="24.0">
+
+  <path android:fillColor="#FFF" android:pathData="M6,19h4L10,5L6,5v14zM14,5v14h4L18,5h-4z"/>
+</vector>

library/src/main/res/drawable-v21/exo_controls_play.xml

+<vector xmlns:android="http://schemas.android.com/apk/res/android"
+  android:width="32dp"
+  android:height="32dp"
+  android:viewportHeight="24.0"
+  android:viewportWidth="24.0">
+
+  <path android:fillColor="#FFF" android:pathData="M8,5v14l11,-7z"/>
+</vector>

library/src/main/res/drawable-v21/exo_controls_previous.xml

+<vector xmlns:android="http://schemas.android.com/apk/res/android"
+  android:width="32dp"
+  android:height="32dp"
+  android:viewportHeight="24.0"
+  android:viewportWidth="24.0">
+
+  <path android:fillColor="#FFF" android:pathData="M6,6h2v12L6,18zM9.5,12l8.5,6L18,6z"/>
+</vector>

library/src/main/res/drawable-v21/exo_controls_rewind.xml

+<vector xmlns:android="http://schemas.android.com/apk/res/android"
+  android:width="32dp"
+  android:height="32dp"
+  android:viewportHeight="24.0"
+  android:viewportWidth="24.0">
+
+  <path android:fillColor="#FFF" android:pathData="M11,18L11,6l-8.5,6 8.5,6zM11.5,12l8.5,6L20,6l-8.5,6z"/>
+</vector>