Fix: fragmentSize is below the minimum (#6238)

noticed logs " CacheDataSink: fragmentSize is below the minimum recommended value of 2097152. This may cause poor cache performance.

this fixes the issue by using ExoPlayers MIN_RECOMMENDED_FRAGMENT_SIZE. Unfortunately that field is private:
aeb306a164/library/core/src/main/java/com/google/android/exoplayer2/upstream/cache/CacheDataSink.java (L123)

app/src/main/java/org/schabi/newpipe/player/helper/PlayerHelper.java

@@ -297,7 +297,7 @@ public final class PlayerHelper {
     }
 
     public static long getPreferredFileSize() {
-        return 512 * 1024L;
+        return 2 * 1024 * 1024L; // ExoPlayer CacheDataSink.MIN_RECOMMENDED_FRAGMENT_SIZE
     }
 
     /**