add display listener class

+package dev.steenbakker.mobile_scanner
+
+import android.hardware.display.DisplayManager
+
+/**
+ * This class will listen for display changes
+ * and executes `onUpdateResolution` when that happens.
+ */
+class MobileScannerDisplayListener(
+    private val onUpdateResolution: () -> Unit
+) : DisplayManager.DisplayListener {
+    override fun onDisplayAdded(displayId: Int) {}
+
+    override fun onDisplayRemoved(displayId: Int) {}
+
+    override fun onDisplayChanged(displayId: Int) {
+        onUpdateResolution()
+    }
+}