AccountObserver listeners were being triggered correctly, however, during every time
we open HomeFragment, home menu gets re-created, which causes us to re-run the initialization
block. Before this patch, the init block would never touch the account manager.
After this patch, it will query it if account manager has already been initialized.
This replaces the StartupTaskManager we had with a more general class.
New implementation is a thread-safe "gated task executor", which either
runs the task right away if it's marked as 'ready', or queries it to be
executed later on.
This ability to either execute or queue a task will be useful later on in the
commit series.
Grisha Kruglov