autocomplete [nfc]: Use new mixin for ComposeAutocomplete's state

As with other uses of PerAccountStoreAwareStateMixin, we expect the
benefit to be noticeable after our planned implementation of #185,
"Start new event queue when old one has expired".

At that time, this should mean that an expiring event queue won't
break autocomplete: if the lifetime of your autocomplete intent
spans across a queue-renewal, you can continue to edit your query
after the renewal, and the app will ask the new store for the query
results, instead of asking the old one.

lib/widgets/autocomplete.dart

@@ -26,15 +26,21 @@ class ComposeAutocomplete extends StatefulWidget {
   State<ComposeAutocomplete> createState() => _ComposeAutocompleteState();
 }
 
-class _ComposeAutocompleteState extends State<ComposeAutocomplete> {
+class _ComposeAutocompleteState extends State<ComposeAutocomplete> with PerAccountStoreAwareStateMixin<ComposeAutocomplete> {
   MentionAutocompleteView? _viewModel; // TODO different autocomplete view types
 
+  void _initViewModel() {
+    final store = PerAccountStoreWidget.of(context);
+    _viewModel = MentionAutocompleteView.init(store: store, narrow: widget.narrow)
+      ..addListener(_viewModelChanged);
+  }
+
   void _composeContentChanged() {
     final newAutocompleteIntent = widget.controller.autocompleteIntent();
     if (newAutocompleteIntent != null) {
-      final store = PerAccountStoreWidget.of(context);
-      _viewModel ??= MentionAutocompleteView.init(store: store, narrow: widget.narrow)
-        ..addListener(_viewModelChanged);
+      if (_viewModel == null) {
+        _initViewModel();
+      }
       _viewModel!.query = newAutocompleteIntent.query;
     } else {
       if (_viewModel != null) {
@@ -51,6 +57,16 @@ class _ComposeAutocompleteState extends State<ComposeAutocomplete> {
     widget.controller.addListener(_composeContentChanged);
   }
 
+  @override
+  void onNewStore() {
+    if (_viewModel != null) {
+      final query = _viewModel!.query;
+      _viewModel!.dispose();
+      _initViewModel();
+      _viewModel!.query = query;
+    }
+  }
+
   @override
   void didUpdateWidget(covariant ComposeAutocomplete oldWidget) {
     super.didUpdateWidget(oldWidget);