fix: Fix view-not-found race condition in C++ code (#511)

* Add custom `onViewReady` event to get layout

`componentDidMount` is async, so the native view _might_ not exist yet causing a race condition in the `setFrameProcessor` code.

This PR fixes this by calling `setFrameProcessor` only after the native view has actually mounted, and to ensure that I created a custom event that fires at that point.

* Update CameraView.swift

android/src/main/java/com/mrousavy/camera/CameraViewManager.kt

@@ -55,6 +55,7 @@ class CameraViewManager(reactContext: ReactApplicationContext) : SimpleViewManag
 
   override fun getExportedCustomDirectEventTypeConstants(): MutableMap<String, Any>? {
     return MapBuilder.builder<String, Any>()
+      .put("cameraViewReady", MapBuilder.of("registrationName", "onViewReady"))
       .put("cameraInitialized", MapBuilder.of("registrationName", "onInitialized"))
       .put("cameraError", MapBuilder.of("registrationName", "onError"))
       .put("cameraPerformanceSuggestionAvailable", MapBuilder.of("registrationName", "onFrameProcessorPerformanceSuggestionAvailable"))