Android app integration (#547)

Summary: Pull Request resolved: https://github.com/pytorch/executorch/pull/547

Test Plan:
To repro, use Android Studio to open examples/demo-apps/android/ExecuTorchDemo.

Follow https://www.internalfb.com/code/fbsource/[D49790694-V13]/xplat/executorch/examples/demo-apps/android/ExecuTorchDemo/README.md

Then
```
cp dl3_xnnpack_fp32.pte ic3_xnnpack_fp32.pte examples/demo-apps/android/ExecuTorchDemo/app/src/main/assets/
```

Also

```
cp libexecutorchdemo.so examples/demo-apps/android/ExecuTorchDemo/app/src/main/jniLibs/arm64-v8a/
```

Reviewed By: cccclai

Differential Revision: D49790694

Pulled By: kirklandsign

fbshipit-source-id: 9022e266ea5d2d3944d879ffb682a03e3fe82df7