fixing const/non-const issues in preparation for libcxx rebase

Change-Id: I8528edd3af4a801f29c17f1f54c44be1a1a0b563
diff --git a/cmds/installd/otapreopt.cpp b/cmds/installd/otapreopt.cpp
index ac511ec..3f76770 100644
--- a/cmds/installd/otapreopt.cpp
+++ b/cmds/installd/otapreopt.cpp
@@ -506,8 +506,8 @@
                              const char *apk_path,
                              const char *instruction_set) {
     // TODO: Insert B directory.
-    char *file_name_start;
-    char *file_name_end;
+    const char *file_name_start;
+    const char *file_name_end;
 
     file_name_start = strrchr(apk_path, '/');
     if (file_name_start == nullptr) {