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/CameraView.kt

@@ -108,6 +108,7 @@ class CameraView(context: Context, private val frameProcessorThread: ExecutorSer
     }
 
   // private properties
+  private var isMounted = false
   private val reactContext: ReactContext
     get() = context as ReactContext
 
@@ -266,6 +267,10 @@ class CameraView(context: Context, private val frameProcessorThread: ExecutorSer
   override fun onAttachedToWindow() {
     super.onAttachedToWindow()
     updateLifecycleState()
+    if (!isMounted) {
+      isMounted = true
+      invokeOnViewReady()
+    }
   }
 
   override fun onDetachedFromWindow() {