Use Context.getFilesDir as a backup dex location

On some devices it seems impossible to read or write the application
data directory. There, creating code_cache at the proper location is
impossible. In this case fallback to the 'files' directory. This may
lead to not cleaning the useless extracted secondary dex files if one
such devices is ever updated to L.

Bug: https://code.google.com/p/android/issues/detail?id=79388
Change-Id: I4b6725572f10fd511992dc8a5043d2f135abd3a5