[ae/darwin] we don't need another mutex when we already have one
[DROID][AE] Don't crash on passthrough.

The AudioTrack sink only ever wants S16LE.
Merge pull request #4279 from t-nelson/droid_passthrough_fix

[DROID][AE] Don't crash on passthrough.
GroupUtils: grouping an empty list is not an error
video library: show the "Select movie set" dialog even if there are no sets to choose from
CGUIDialogSelect: focus the extra button (if available) when the selection list is empty