Improve push notifications (#4896)

Besides the refactoring these improvements:
* Track last push distributor and reset settings and subscription on any
incompatible change (ie. uninstall)
* Only update (push) notification settings on server if needed
* Allow to only fetch notifications for one account (the one for which a
push message was received)

This is (also) the revival of
https://github.com/tuskyapp/Tusky/pull/3642

It's not really well tested so far. (Ie. with two or more accounts or
two or more push providers.)

app/src/test/java/com/keylesspalace/tusky/MainActivityTest.kt

@@ -106,7 +106,9 @@ class MainActivityTest {
                 on { areNotificationsEnabled() } doReturn true
             },
             mock(),
+            mock(),
             context,
+            mock(),
         )
 
         notificationService.createNotificationChannelsForAccount(accountEntity)