[TIMOB-24536] Android: Implement getExternalFilesDir in Ti.Filesystem

Currently, Titanium SDK apps can get paths save data in applicationDataDirectory or externalDataStorage (which seems to go to the SD card if one is installed). Android provides primary external storage for apps. They can retrieve that path using the getExternalFilesDir function. See https://developer.android.com/reference/android/content/Context.html#getExternalFilesDir(java.lang.String) It would be helpful for apps to have access to this directory so they can save documents that are visible to the user (to transfer documents to another device) or to sync using apps like FolderSync. Documents saved in the private app data directory are not visible outside of the app itself. Please add a field to Ti.Filesystem to provide this directory.