Added support for android local storage via set_dir! macro

[pvg13]

Hi!

I added a small helper function to locate the Android directory for the storage SDK: data_directory.

It works as follows:

1. Sets up a communication channel between the main Rust thread and the Android execution thread.
2. Dispatches a closure to run on the Android/JVM thread where it has access to the Java environment (JNIEnv).
3. Calls Java methods (getFilesDir and getAbsolutePath) to find where the app is allowed to save files.
4. Converts the result from a Java String to a Rust PathBuf.
5. Blocks and waits until the path is received, then returns it.

In the set_dir! macro if target_os = "android" it will make a call to the data_directory function to figure out the correct path for the app.

I don't know if this is the most elegant solution, maybe it's better to give the information via dx or something similar.

This probably also fixes: #99

[ealmloff]

Thanks!