Don't automatically backup settings in Android demo apps (#5822)

Summary:
When re-installing the android demo apps, the OS caches and re-populates values in the settings pane.

I think this might cause some confusion for people trying out the demos but who aren't experienced native app devs (like me 🙂).

specifically for me, the settings caching made me think that old files that i had pushed, removed, and then replaced with other files were still on the device somehow. eg, i replaced a `tokenizer.bin` with a `tokenizer.model`, and the settings still said `tokenizer.bin`... this made me think maybe the old tokenizer was still floating around in app storage or something.

to change the behavior is 1 loc for each demo app. in `examples/demo-apps/android/*Demo/app/src/main/AndroidManifest.xml`,

```
-        android:allowBackup="true"
+        android:allowBackup="false"
```

i made a PR for personal glory but no worries if you want to do something else.

Pull Request resolved: https://github.com/pytorch/executorch/pull/5822

Reviewed By: cccclai

Differential Revision: D64434744

Pulled By: kirklandsign

fbshipit-source-id: 3c0a764b6e403eaec9ec0e1fc0ff09f4beb15ac8