- Declare it in shared utils.h for thumbnailer.c
- Fix a memleak related to jstring->string in getLengthFromFile()
- Create a java helper to get iomx preference and use it

Code removed:
- add_media_codec_options() : merged into new function
- static bool : we fetch the preference directly
- No need to give iomx preference to nativeInit()
- No need to reinit libvlc on iomx preference change
- No need to monitor those changes
- No need for reinit() function

Notes:

We could add a new boolean field, continue to monitor preference changes
and update that field; then access this field from JNI instead of a method.

Since android works well with URIs we should use media_new_location()
(Need to check all code paths to this function)