Use 'keystatuseschange' events and support @group.

Use 'keystatuseschange' events from EME together with cenc:default_KID
from the MPD so StreamVideoSource can determine if the key system
can/will decrypt a stream before it attempts to play it. This enables
the Player to safely (and automatically) switch between streams that
are encrypted with different keys.

Support AdaptationSet @group, which is the preferred approach to safely
use multiple encryption keys over multiple Representations.

* Squash AdaptationSets from the same group into one StreamSetInfo.
  This enables support for @group without introducing special case code
  into StreamVideoSource and EmeManager.
* Fire 'trackchanged' events when tracks becomes available/unavailable
  either from application restrictions or key status changes.
* Pull key IDs out of DrmInfo and put them into StreamInfo so key status
  changes can drive per stream availability instead of per StreamSet
  (AdaptationSet) availability.

Closes #67
Issue #160

Change-Id: Ife0814deb65715923a572b45880137a99b378035

lib/util/string_utils.js

@@ -50,3 +50,16 @@ shaka.util.StringUtils.fromBase64 = function(str) {
   return window.atob(str.replace(/-/g, '+').replace(/_/g, '/'));
 };
 
+
+/**
+ * Separates every 4 characters by a space.
+ * @param {string} str
+ * @return {string}
+ */
+shaka.util.StringUtils.formatHexString = function(str) {
+  return str.split('').reduce(
+      function(acc, ch, i) {
+        return acc + (i && (i % 4 == 0) ? ' ' + ch : ch);
+      });
+};
+