Address const issues in preparation for libcxx rebase.

Change-Id: I044a41c6876a008f3f5011e2649d8557d702e802
diff --git a/media/libstagefright/MediaCodecList.cpp b/media/libstagefright/MediaCodecList.cpp
index 0fb5072..08bb45d 100644
--- a/media/libstagefright/MediaCodecList.cpp
+++ b/media/libstagefright/MediaCodecList.cpp
@@ -182,7 +182,7 @@
 
 void MediaCodecList::parseTopLevelXMLFile(const char *codecs_xml, bool ignore_errors) {
     // get href_base
-    char *href_base_end = strrchr(codecs_xml, '/');
+    const char *href_base_end = strrchr(codecs_xml, '/');
     if (href_base_end != NULL) {
         mHrefBase = AString(codecs_xml, href_base_end - codecs_xml + 1);
     }