Change dex_location_ from const char* to std::string.

Because we tend to use the dex_location_ as an std::string fairly
often.

Change-Id: Ida9624c9cb12c8ad30577146f1e6e97e25edc155
diff --git a/runtime/oat_file_assistant.h b/runtime/oat_file_assistant.h
index 664db98..f781532 100644
--- a/runtime/oat_file_assistant.h
+++ b/runtime/oat_file_assistant.h
@@ -369,9 +369,7 @@
   // remaining lifetime of the OatFileAssistant object.
   ScopedFlock flock_;
 
-  // In a properly constructed OatFileAssistant object, dex_location_ should
-  // never be null.
-  const char* dex_location_ = nullptr;
+  std::string dex_location_;
 
   // In a properly constructed OatFileAssistant object, isa_ should be either
   // the 32 or 64 bit variant for the current device.