Address const issues in preparation for libcxx rebase.

Change-Id: I389e585b11647763f7b4f4dbf1181acc85ba36d4
diff --git a/media/libstagefright/DataURISource.cpp b/media/libstagefright/DataURISource.cpp
index 2a61c3a..3dc345f 100644
--- a/media/libstagefright/DataURISource.cpp
+++ b/media/libstagefright/DataURISource.cpp
@@ -28,7 +28,7 @@
         return NULL;
     }
 
-    char *commaPos = strrchr(uri, ',');
+    const char *commaPos = strrchr(uri, ',');
 
     if (commaPos == NULL) {
         return NULL;