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:

• Allow permissions and services to be specified for Android builds kollivier/pyeverywhere#21
• Fixes to Android integrations to prevent errors kolibri#6758

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

[kollivier]

w00t!

[jamalex]

Just wait... it escalated from there! Additional PR incoming.

[kollivier]

Just flagging that I'm thinking of moving some of the Android code for creating the foreground service into pyeverywhere, and hopefully formalizing the concept of a service in the library. I think it's best to just focus on getting everything working first though, and then we can look at how best to design the longer-term approach. :)