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.