Android: Add Ti.Filesystem.externalCacheDirectory support

Summary:
On Android and iOS, we offer an applicationCacheDirectory property used to write temporary files to internal storage.

Android supports a sandboxed cache directory for the app on external storage, which typically provides more storage space. Unlike externalStorageDirectory, the external cache directory is not automatically backed-up by the system. This makes it a good location for LARGE temporary files.

We should expose this Android-only feature via the following property.
Ti.Filesystem.externalCacheDirectory

Note 1:
You do NOT need the following permissions to read/write to the external cache folder.

• READ_EXTERNAL_STORAGE
• WRITE_EXTERNAL_STORAGE

Note 2:
You do NOT need to use the following APIs before reading/writing to this folder.

• Ti.Filesystem.hasStoragePermissions()
• Ti.Filesystem.requestStoragePermissions()

Note 3:
You do need to check the isExternalStoragePresent() method to see if external storage is mounted before using this folder.

Example:

if (Ti.Filesystem.isExternalStoragePresent()) {

	const myFile = Ti.Filesystem.getFile(Ti.Filesystem.externalCacheDirectory, 'MyFile.txt');

	myFile.write('Hello World');

}