Update default max buffer in DefaultLoadControl.

The value is increased to 50 seconds. With that the player can better handle
short network problems. This does NOT increase the maximum memory used as we
still apply the seperate DEFAULT_TARGET_BUFFER_BYTES.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=188335603

RELEASENOTES.md

@@ -4,6 +4,7 @@
 
 * Moved initial bitrate estimate from `AdaptiveTrackSelection` to
   `DefaultBandwidthMeter`.
+* Updated default max buffer length in `DefaultLoadControl`.
 * UI components:
   * Add support for listening to `AspectRatioFrameLayout`'s aspect ratio update
     ([#3736](https://github.com/google/ExoPlayer/issues/3736)).

library/core/src/main/java/com/google/android/exoplayer2/DefaultLoadControl.java

@@ -37,7 +37,7 @@ public class DefaultLoadControl implements LoadControl {
   /**
    * The default maximum duration of media that the player will attempt to buffer, in milliseconds.
    */
-  public static final int DEFAULT_MAX_BUFFER_MS = 30000;
+  public static final int DEFAULT_MAX_BUFFER_MS = 50000;
 
   /**
    * The default duration of media that must be buffered for playback to start or resume following a