Treat SqueezerItems with getId() == null as identical.

Fixes a problem when connecting to a server with no currently playing
song -- the "current" song and "new" song weren't comparing as equal,
resulting in repeated calls to updateOngoingNotification().

Add unit tests to verify that equals() works as expected.
Merge remote-tracking branch 'kaaholst/actionbar' into actionbar
Fix an NPE when togglePausePlay() is called with no connected players.
Merge branch 'actionbar' of https://github.com/nikclayton/android-squeezer into actionbar
Add a "wrapper" task, expected by Android Studio.