bug fix: uncaught exception when CastSession ends in "CAST_ONLY" mode

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issue:
  * uncaught exception leads to crash

steps to reproduce:
  * pause "VideoActivity"
  * click "stop" on TV remote control to end CastSession

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CastPlayer:
===========

onSessionEnded()
 -> setRemoteMediaClient(null)
    -> sessionAvailabilityListener.onCastSessionUnavailable()

VideoManager:
=============

onCastSessionUnavailable()
 -> release(false)
    -> release_castPlayer(false)
       -> castPlayer.release()

CastPlayer:
===========

release()
 -> sessionManager.endCurrentSession(false)
    -> ERROR: because this entire chain of events started when the current session ended..

solution:
=========

in: VideoManager
in: -> onCastSessionUnavailable
old:   -> release(false)
new:   -> release(true)  // retain_cast_session

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old:
====

  @OverRide
  public void onCastSessionUnavailable() {
    if (castPlayer == null) return;

    if (playbackMode == PlaybackMode.CAST_ONLY) {
      setPlaybackMode(PlaybackMode.RELEASED);
    }
    else {
      setCurrentPlayer(exoPlayer);
    }
  }

new:
====

  @OverRide
  public void onCastSessionUnavailable() {
    if (castPlayer == null) return;

    if (playbackMode == PlaybackMode.CAST_ONLY) {
      setPlaybackMode(PlaybackMode.RELEASED_ALL_BUT_CAST_SESSION);
    }
    else {
      setCurrentPlayer(exoPlayer);
    }
  }

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WebCast/src/main/java/com/github/warren_bank/webcast/exoplayer2/PlayerManager.java

@@ -415,7 +415,7 @@ public void onCastSessionUnavailable() {
     if (castPlayer == null) return;
 
     if (playbackMode == PlaybackMode.CAST_ONLY) {
-      setPlaybackMode(PlaybackMode.RELEASED);
+      setPlaybackMode(PlaybackMode.RELEASED_ALL_BUT_CAST_SESSION);
     }
     else {
       setCurrentPlayer(exoPlayer);

constants.gradle

@@ -1,6 +1,6 @@
 project.ext {
-    releaseVersionCode = 004050016
-    releaseVersion = '004.05.00-16API'
+    releaseVersionCode = 004060016
+    releaseVersion = '004.06.00-16API'
     minSdkVersion = 16
     targetSdkVersion = 28
     compileSdkVersion = 28