android: When opening a notification, "back" should not leave the account's context

[chrisbobbe]

From beta feedback.

If the app is showing UI for Account A and you open a notification for Account B, we push a message-list route for Account B onto the nav stack that's supposed to belong to Account A. This means the "back" button will take you out of Account B's context, which can be confusing.

When we navigate on opening a notification, we should switch to a new nav stack for the notification's account (if different) before pushing the message-list route.

(The iOS counterpart is covered by #1147, for navigating at all when a notification is tapped.)

Implementation

Probably we want to call HomePage.navigate—

  /// Navigate to [HomePage], ensuring that its route is at the root level.
  static void navigate(BuildContext context, {required int accountId}) {
    final navigator = Navigator.of(context);
    navigator.popUntil((route) => route.isFirst);
    unawaited(navigator.pushReplacement(
      HomePage.buildRoute(accountId: accountId)));
  }

—and then push the result of MessageListPage.buildRoute after that.