fix: Fix Android focus not using correct focus point (#958)

This commit fixes #758. I was having the same issue and looked into it a bit. I found
[this StackOverflow answer](https://stackoverflow.com/a/60585382) which described a
solution to the same problem. Rather than manually calculate the focus point, we can
get the PreviewView to do it for us. This fixes the issue because the PreviewView
factors in any scaling or resizing of the view on the screen, which we weren't doing
before. The only potential issue is that this needs to run on the UI thread
(which is what the `withContext` is doing), but I've tested it with frame processors
enabled and disabled, and have found no issues in either case.

android/src/main/java/com/mrousavy/camera/CameraView+Focus.kt

@@ -1,9 +1,9 @@
 package com.mrousavy.camera
 
 import androidx.camera.core.FocusMeteringAction
-import androidx.camera.core.SurfaceOrientedMeteringPointFactory
 import com.facebook.react.bridge.ReadableMap
 import kotlinx.coroutines.guava.await
+import kotlinx.coroutines.withContext
 import java.util.concurrent.TimeUnit
 
 suspend fun CameraView.focus(pointMap: ReadableMap) {
@@ -16,8 +16,11 @@ suspend fun CameraView.focus(pointMap: ReadableMap) {
   val x = pointMap.getDouble("x") * dpi
   val y = pointMap.getDouble("y") * dpi
 
-  val factory = SurfaceOrientedMeteringPointFactory(this.width.toFloat(), this.height.toFloat())
-  val point = factory.createPoint(x.toFloat(), y.toFloat())
+  // Getting the point from the previewView needs to be run on the UI thread
+  val point = withContext(coroutineScope.coroutineContext) {
+    previewView.meteringPointFactory.createPoint(x.toFloat(), y.toFloat());
+  }
+
   val action = FocusMeteringAction.Builder(point, FocusMeteringAction.FLAG_AF or FocusMeteringAction.FLAG_AE)
     .setAutoCancelDuration(5, TimeUnit.SECONDS) // auto-reset after 5 seconds
     .build()

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

@@ -121,7 +121,7 @@ class CameraView(context: Context, private val frameProcessorThread: ExecutorSer
   private val cameraExecutor = Executors.newSingleThreadExecutor()
   internal val takePhotoExecutor = Executors.newSingleThreadExecutor()
   internal val recordVideoExecutor = Executors.newSingleThreadExecutor()
-  private var coroutineScope = CoroutineScope(Dispatchers.Main)
+  internal var coroutineScope = CoroutineScope(Dispatchers.Main)
 
   internal var camera: Camera? = null
   internal var imageCapture: ImageCapture? = null