Move permission code into permissions class

+package dev.steenbakker.mobile_scanner
+
+import android.Manifest.permission
+import android.app.Activity
+import android.content.pm.PackageManager
+import androidx.core.app.ActivityCompat
+import androidx.core.content.ContextCompat
+import io.flutter.plugin.common.PluginRegistry.RequestPermissionsResultListener
+
+/**
+ * This class handles the camera permissions for the Mobile Scanner.
+ */
+class MobileScannerPermissions {
+    companion object {
+        const val CAMERA_ACCESS_DENIED = "CameraAccessDenied"
+        const val CAMERA_ACCESS_DENIED_MESSAGE = "Camera access permission was denied."
+        const val CAMERA_PERMISSIONS_REQUEST_ONGOING = "CameraPermissionsRequestOngoing"
+        const val CAMERA_PERMISSIONS_REQUEST_ONGOING_MESSAGE = "Another request is ongoing and multiple requests cannot be handled at once."
+
+        /**
+         * When the application's activity is [androidx.fragment.app.FragmentActivity], requestCode can only use the lower 16 bits.
+         * @see androidx.fragment.app.FragmentActivity.validateRequestPermissionsRequestCode
+         */
+        const val REQUEST_CODE = 0x0786
+    }
+
+    interface PermissionsRegistry {
+        @SuppressWarnings("deprecation")
+        fun addListener(handler: RequestPermissionsResultListener)
+    }
+
+    interface ResultCallback {
+        fun onResult(errorCode: String?, errorDescription: String?)
+    }
+
+    private var listener: RequestPermissionsResultListener? = null
+
+    fun getPermissionListener(): RequestPermissionsResultListener? {
+        return listener
+    }
+
+    private var ongoing: Boolean = false
+
+    fun hasCameraPermission(activity: Activity) : Int {
+        val hasPermission = ContextCompat.checkSelfPermission(
+            activity,
+            permission.CAMERA,
+        ) == PackageManager.PERMISSION_GRANTED
+
+        return if (hasPermission) {
+            1
+        } else {
+            0
+        }
+    }
+
+    fun requestPermission(activity: Activity,
+                          permissionsRegistry: PermissionsRegistry,
+                          callback: ResultCallback) {
+        if (ongoing) {
+            callback.onResult(
+                CAMERA_PERMISSIONS_REQUEST_ONGOING, CAMERA_PERMISSIONS_REQUEST_ONGOING_MESSAGE)
+            return
+        }
+
+        if(hasCameraPermission(activity) == 1) {
+            // Permissions already exist. Call the callback with success.
+            callback.onResult(null, null)
+            return
+        }
+
+        if(listener == null) {
+            // Keep track of the listener, so that it can be unregistered later.
+            listener = MobileScannerPermissionsListener(
+                object: ResultCallback {
+                    override fun onResult(errorCode: String?, errorDescription: String?) {
+                        ongoing = false
+                        callback.onResult(errorCode, errorDescription)
+                    }
+                }
+            )
+            permissionsRegistry.addListener(listener)
+        }
+
+        ongoing = true
+        ActivityCompat.requestPermissions(
+            activity,
+            arrayOf(permission.CAMERA),
+            REQUEST_CODE
+        )
+    }
+}
+
+/**
+ * This class handles incoming camera permission results.
+ */
+@SuppressWarnings("deprecation")
+private class MobileScannerPermissionsListener(
+    private val resultCallback: MobileScannerPermissions.ResultCallback,
+): RequestPermissionsResultListener {
+    // There's no way to unregister permission listeners in the v1 embedding, so we'll be called
+    // duplicate times in cases where the user denies and then grants a permission. Keep track of if
+    // we've responded before and bail out of handling the callback manually if this is a repeat
+    // call.
+    private var alreadyCalled: Boolean = false
+
+    override fun onRequestPermissionsResult(
+        requestCode: Int,
+        permissions: Array<out String>,
+        grantResults: IntArray
+    ): Boolean {
+        if (alreadyCalled || requestCode != MobileScannerPermissions.REQUEST_CODE) {
+            return false
+        }
+
+        alreadyCalled = true
+
+        // grantResults could be empty if the permissions request with the user is interrupted
+        // https://developer.android.com/reference/android/app/Activity#onRequestPermissionsResult(int,%20java.lang.String[],%20int[])
+        if (grantResults.isEmpty() || grantResults[0] != PackageManager.PERMISSION_GRANTED) {
+            resultCallback.onResult(
+                MobileScannerPermissions.CAMERA_ACCESS_DENIED,
+                MobileScannerPermissions.CAMERA_ACCESS_DENIED_MESSAGE)
+        } else {
+            resultCallback.onResult(null, null)
+        }
+
+        return true
+    }
+}