presence: Dispose Presence object when store disposed

I happened to notice this message getting printed repeatedly in the
debug logs (reformatted a bit):

    [ERROR:flutter/runtime/dart_vm_initializer.cc(40)] Unhandled Exception:
      NetworkException: HTTP request failed. Client is already closed.
      (ClientException: HTTP request failed. Client is already closed.,
       uri=https://chat.zulip.org/api/v1/users/me/presence)
    #0      ApiConnection.send (package:zulip/api/core.dart:175)
    <asynchronous suspension>
    #1      Presence._maybePingAndRecordResponse (package:zulip/model/presence.dart:93)
    <asynchronous suspension>
    #2      Presence._poll (package:zulip/model/presence.dart:121)
    <asynchronous suspension>

That'd be a symptom of an old Presence continuing to run its polling
loop after the ApiConnection has been closed, which happens when the
PerAccountStore is disposed.  Looks like when we introduced Presence
in 5d43df2 (#1619), we forgot to call its `dispose` method.
Fix that now.

The presence model doesn't currently have any tests.  So rather than
try to add a test for just this, we'll leave it as something to
include when we write those tests, #1620.

Author: gnprice

lib/model/store.dart

@@ -878,6 +878,7 @@ class PerAccountStore extends PerAccountStoreBase with
     recentDmConversationsView.dispose();
     unreads.dispose();
     _messages.dispose();
+    presence.dispose();
     typingStatus.dispose();
     typingNotifier.dispose();
     updateMachine?.dispose();