[Notification] Make constructor explicit
[Notification] Fix leaking ContextAPI object
[Notification] Remove locking from NotificationManager in mobile

Locking was necessary because each instance was running in a separated
thread, but now all of them share the same thread, so it's safe to
access shared state without locking.
[Notification] Fix wrong loop in OnDetailedChanged

The loop was missing incrementing the iterator. This led to an infinite
loop, and then to crash in the extension process. This patch experiments
with std::find_if to avoid having to write the raw loop at all.
Merge pull request #150 from cmarcelo/notification-fixes

Notification fixes