Switch Kolibri server start to a separate persistent foreground service

[jamalex]

• Some general refactors and cleanup -- mostly swamped by the subsequent changes in the switch to using a service.
• Moved the server start code to a P4A service so that it's decoupled from the webview activity.
• Followed this recipe to make the background service a foreground service, to prevent it from being killed.

This relies on the following changes to both pyeverywhere and kolibri:

• https://github.com/kollivier/pyeverywhere/pull/21/
• https://github.com/learningequality/kolibri/pull/6758

(May be easiest to review main.py by just viewing the new version, rather than the diff)