Enable Push by default

[rfc2822]

Currently, the "Push provider" setting is set to None by default.

I think Push is stable enough to enable it by default. As all traffic is encrypted by default, this shouldn't cause any privacy problems.

However to which provider, especially if there are multiple ones? To the first one in the list, or to the most stable one (which would surely by FCM if it's available)?

And how do we handle changes of installed providers? We should probably listen for changes and then enable/disable automatically.

Are there other things which we should have in mind when implementing that?

[devvv4ever]

For Managed DAVx5 this should be a setting I'd recommend. I would however set the default value to FCM, since more or less all our customers use Managed DAVx5 from "Google Play (for Work)" and FCM is available there most likely.

[ArnyminerZ]

I'd set FCM by default to GMS-based-builds (Play Store / Samsung?), and for the rest, select the first provided available.

If the selected provider is disabled/uninstalled, select the first available one, or disable push completely if there are none. If there were none, and now one is installed (can we detect that?) that one would be enabled now automatically.