Issue 1501: Use unique temporary directory name when importing.

[Issue 1501](https://code.google.com/p/ankidroid/issues/detail?id=1501)

While importing, a temporary directory is used to extract the apkg, then deleted when done. For some reason that I couldn't work out, it's not possible to recreate this directory again without terminating AnkiDroid. Thus, subsequent imports fail. 

This is a workaround to use a UUID suffix in the temporary directory name to guarantee it's unique every time.