fix(DRM): Fix drm choice when the user provide a server in the manifest

Author: avelad

lib/drm/drm_engine.js

@@ -1083,7 +1083,17 @@ shaka.drm.DrmEngine = class {
 
     // If we have configured preferredKeySystems, choose a preferred keySystem
     // if available.
-    for (const preferredKeySystem of this.config_.preferredKeySystems) {
+    let preferredKeySystems = this.config_.preferredKeySystems;
+    if (!preferredKeySystems.length) {
+      // If there is no preference set and we only have one server license, we
+      // use this as preference. This is used to override manifests on those
+      // that have the embedded license and the browser supports multiple DRMs.
+      const servers = shaka.util.MapUtils.asMap(this.config_.servers);
+      if (servers.size == 1) {
+        preferredKeySystems = servers.keys();
+      }
+    }
+    for (const preferredKeySystem of preferredKeySystems) {
       for (const variant of variants) {
         const decodingInfo = variant.decodingInfos.find((decodingInfo) => {
           return decodingInfo.supported &&