Notification tweaks

[rtibbles]

• Makes a small tweak to how the notificationId is accessed across threads, to ensure that we are always initializing and referencing a thread local variable for the notificationId - possible that this could have caused confusion across multiple synchronous worker threads as to which notificationId was meant to managed
• Adds 5 second delay to clearing of notifications when called. This should ensure that we don't rapidly display and hide ids in the case of slower devices, and in the case of faster devices, that the notification has had a chance to be created before we try to hide it!

Debug build of APK will be here: https://github.com/learningequality/kolibri-installer-android/actions/runs/6930362719

Note that this is built using the Kolibri beta 8 release, so this will exhibit previously logged syncing issues and is not a cause for new concern - this only fixes issues with notifications!

Fixes #174

[radinamatic]

Not necessarily a blocker since it's limited to just some devices, but the flood is still happening.

https://github.com/learningequality/kolibri-installer-android/assets/1457929/66b78a0a-57de-401d-adc8-ff5cd97d4069