feat: handle android back button on android (#1656)

packages/mobile/src/AndroidBackHandlerService.ts

@@ -0,0 +1,21 @@
+import { AbstractService, InternalEventBus, ReactNativeToWebEvent } from '@standardnotes/snjs'
+import { BackHandler, NativeEventSubscription } from 'react-native'
+
+export class AndroidBackHandlerService extends AbstractService<ReactNativeToWebEvent> {
+  private removeListener: NativeEventSubscription
+
+  constructor() {
+    const internalEventBus = new InternalEventBus()
+    super(internalEventBus)
+
+    this.removeListener = BackHandler.addEventListener('hardwareBackPress', () => {
+      void this.notifyEvent(ReactNativeToWebEvent.AndroidBackButtonPressed)
+
+      return true
+    })
+  }
+
+  deinit() {
+    this.removeListener.remove()
+  }
+}