Remove observer pattern

Since handlers can be used in both directions, with inbox and outbox
handlers, I will use this mechanism for notifying the activities. Thus,
I have removed the observer implementation from the model and main
activity.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Clean up main activity a bit

I have added some comments to all those fields and refactored the access
to the controller's inbox handler.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Remove Observer pattern classes

When I removed the observer pattern from the model and activity I forgot
that I had provided my own observer and observable classes, respectively
interfaces. Hence, this commit removes them.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Add outbox handling for issued activity actions

Finally, I am putting the pieces together for handling the messages that
are issued for the controller's outbox. For that I have introduced a new
enumeration called Action which lists the different actions that are
processed by the activity.

For now I have roughly implemented the GCM registration procedure.
Outboxing allows me to set also primitive values, like a boolean, as
object parameter for the message.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Add lock/unlock actions to the status task

Continuing to add outbox inquiries from the tasks I have added now
outbox notifications to the asynchronous task status task. A boolean is
required to indicate whether the detection is currently active or not.
Based on that the the activity elements are locked or unlocked in a
different way.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>