Added automatic push distributor selection

[ArnyminerZ]

Purpose

See #1737, but basically, Push is basically stable enough to at least select a distributor by default.

In any case, if one doesn't trust it, the server admin should simply not install support for push on the server, and the distributor will be unused.

Short description

• Added automatic push distributor selection
  • Added a new injectable class called DistributorPreferences. It contains a list of package names which will define what the preference for each distributor is.
  • For OSE, no distributor has preference.
  • For Play Store builds we might want to pre-select FCM for example, or for managed. This can be set with the DistributorPreferences implementation.
• Added a new settings entry which will be set to true whenever the user manually disables push (because it will be enabled by default). If this setting is true, no push distributor will be automatically selected.

We still have to add the individual implementations for each flavor on bitfireAt/davx5.

Checklist

• The PR has a proper title, description and label.
• I have self-reviewed the PR.
• I have added documentation to complex functions and functions that can be used by other modules.
• I have added reasonable tests or consciously decided to not add tests.

[sunkup]

I will change to draft for now and we can reopen when we want to merge it - that is when we know in which version it will be added.

[rfc2822]

I think it will finally be time 😄

As I understand it, the basic idea is to check/update the push distributor whenever the PushRegistrationWorker is run, instead of setting it explicitly from the UI.

This sounds good to me! However some notes.

(Also please fix conflicts)

[rfc2822]

I think this code should go to a seperate PushDistributorManager, because it's not related to the task of updating the subscriptions.

One class should only do one thing:

• PushDistributorManager → manage push distributor
• PushRegistrationManager → manage actual subscriptions

The same applies to the existing PushRegistrationManager.setPushDistributor(), which should also be moved to PushDistributorManager.

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Then the PushRegistrationWorker could

1. pushDistributorManager.update() → make sure distributor is correct and set up,
2. pushRegistrationManager.update() → update subscriptions with selected push distributor.

[ArnyminerZ]

As agreed on the meeting, FCM pre-selection for gplay variant is kept. So this should be ready for now. We can keep it improving later (actual distributor pinging to make sure it's available)

Note: The gplay preference should be established on the private repo