fix: Reset forced subs between loads

Just as we reset text language and role, we should also reset forced
sub state between load() calls.

Discovered while debugging PR #3459.

Change-Id: I22ef29cb6b14e02ee70503595b1303389260a633

lib/player.js

@@ -1844,6 +1844,7 @@ shaka.Player = class extends shaka.util.FakeEventTarget {
 
     this.currentTextLanguage_ = this.config_.preferredTextLanguage;
     this.currentTextRole_ = this.config_.preferredTextRole;
+    this.currentTextForced_ = this.config_.preferForcedSubs;
 
     shaka.Player.applyPlayRange_(this.manifest_.presentationTimeline,
         this.config_.playRangeStart,