Add symptom view component with back button listener

components/cycle-day/symptoms/symptom-view.js

@@ -0,0 +1,13 @@
+import { Component } from 'react'
+import { BackHandler } from 'react-native'
+
+export default class SymptomView extends Component {
+  constructor() {
+    super()
+    this.backHandler = BackHandler.addEventListener('hardwareBackPress', this.save.bind(this))
+  }
+
+  componentWillUnmount() {
+    this.backHandler.remove()
+  }
+}