Investigate better solution for threading model

[lukaspili]

Currently, the android ServiceConnection used to acquire the billing service always returns on the main thread, regardless of the thread that requests the binding.
If the requester thread is not the main thread, I'm using a semaphore to lock that thread, and I'm unlocking it when the billing service is bound.

See BaseObservable.java for the implementation.

Would like to find better solution that doesn't require such hacky thread manipulation.