Relax string comparison in DASH parseContentProtection

... by making it case insensitive and null-tolerant for schemeId (as was before
adding playlist drm data merging).

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=174472123

library/dash/src/main/java/com/google/android/exoplayer2/source/dash/manifest/DashManifestParser.java

@@ -345,13 +345,14 @@ public class DashManifestParser extends DefaultHandler
    */
   protected Pair<String, SchemeData> parseContentProtection(XmlPullParser xpp)
       throws XmlPullParserException, IOException {
-    String schemeIdUri = xpp.getAttributeValue(null, "schemeIdUri");
     String schemeType = null;
     byte[] data = null;
     UUID uuid = null;
     boolean requiresSecureDecoder = false;
 
-    switch (schemeIdUri) {
+    String schemeIdUri = xpp.getAttributeValue(null, "schemeIdUri");
+    if (schemeIdUri != null) {
+      switch (schemeIdUri.toLowerCase()) {
         case "urn:mpeg:dash:mp4protection:2011":
           schemeType = xpp.getAttributeValue(null, "value");
           String defaultKid = xpp.getAttributeValue(null, "cenc:default_KID");
@@ -370,6 +371,7 @@ public class DashManifestParser extends DefaultHandler
         default:
           break;
       }
+    }
 
     do {
       xpp.next();