Better handling of non-US Amazon MP3 Stores

Non-US stores seem to deliver .amz files with a different content type
(audio/x-amzaudio) than the US store (audio/x-amzxml). Refactor this
content type check into a new method, IsAmzContentType, and use that in
the three places where needed. This completes a previous attempt to fix
this issue from gabaug. Hopefully this fixes downloading issues reported
by some users on non-US stores.

Finally, read and set the cookies against all Amazon MP3 domains: com,
co.uk, co.jp, de, and fr. This should fix the "downloader not installed"
issue that some users are seeing on non-US stores.