Fix bufferSizeInSeconds Not Affecting preferredForwardBufferDuration

[w43322]

In the case of a playback stall (as shown below), the interrupted player will wait for the buffer to catch up before resuming playback. The "safe" amount of buffer headroom is supposed to be configurable.

{
  "transportType" : "HTTP Progressive Download",
  "mediaType" : "HTTP Progressive Download",
  "BundleID" : "RedGift",
  "name" : "MEDIA_PLAYBACK_STALL",
  "interfaceType" : "Loopback"
}

Previously, bufferSizeInSeconds had no effect on preferredForwardBufferDuration, causing the player to wait a long time for a large, Apple-configured buffer headroom value. It should instead adhere to the bufferSizeInSeconds configuration in the Player class. This PR fixes the issue.