Enhance DefaultTrackSelector part 1

- Enforce viewport constraints for fixed video track selection.
- Select best fixed video track, not the first one.
- Better handling of video tracks with unknown dimensions.
- Mini bug fix.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=129226593

library/src/main/java/com/google/android/exoplayer2/Format.java

@@ -411,6 +411,14 @@ public final class Format implements Parcelable {
   }
 
   /**
+   * Returns the number of pixels if this is a video format whose {@link #width} and {@link #height}
+   * are known, or {@link #NO_VALUE} otherwise
+   */
+  public int getPixelCount() {
+    return width == NO_VALUE || height == NO_VALUE ? NO_VALUE : (width * height);
+  }
+
+  /**
    * Returns a {@link MediaFormat} representation of this format.
    */
   @SuppressLint("InlinedApi")