[qmf-notifications-plugin] Don't assert if accountId is invalid.

AccountId becomes invalid for service actions when account is removed
while the action is running.
[qmf-notification-plugin] Improve notifications logic

- Beep only if app is on screen.
- Remove notifications when app becomes top most.
Merge pull request #9 from nemomobile/vdv-devel

[qmf-notification-plugin] Improve notifications logic