Android: get external storage directory dynamically through JNI

[JeroMiya]

Currently on android we have "/sdcard/osvr/osvr_server_config.json" as a hard-coded search path for the server config. We should be calling Environment.getExternalStorageDirectory() dynamically at runtime via JNI to get the true path reliably on different android distributions. This requires a JNI_OnLoad function definition for the Android build to get access to the JNI VM in order to make the appropriate calls.