fix: reduce asset_viewer reloads (#20083)

* fix: reduce asset_viewer reloads

* limit the result of watch asset to one

* fix null reference in map thumbnail

* bump hash file limit to 256

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Co-authored-by: shenlong-tanwen <139912620+shalong-tanwen@users.noreply.github.com>
Co-authored-by: Alex <alex.tran1502@gmail.com>

mobile/lib/presentation/widgets/asset_viewer/asset_viewer.state.dart

@@ -7,6 +7,10 @@ class ViewerOpenBottomSheetEvent extends Event {
   const ViewerOpenBottomSheetEvent();
 }
 
+class ViewerReloadAssetEvent extends Event {
+  const ViewerReloadAssetEvent();
+}
+
 class AssetViewerState {
   final int backgroundOpacity;
   final bool showingBottomSheet;