Address const issues in preparation for libcxx rebase.

Change-Id: I069b4c9c2e50569b7cf49b25c8c87e7f36833ab5
diff --git a/media/libstagefright/wifi-display/source/WifiDisplaySource.cpp b/media/libstagefright/wifi-display/source/WifiDisplaySource.cpp
index e26165e..b43802d 100644
--- a/media/libstagefright/wifi-display/source/WifiDisplaySource.cpp
+++ b/media/libstagefright/wifi-display/source/WifiDisplaySource.cpp
@@ -782,7 +782,7 @@
             return;
         }
 
-        char *commaPos = strchr(s, ',');
+        const char *commaPos = strchr(s, ',');
         if (commaPos != NULL) {
             s = commaPos + 1;