Add a mock perf test for llama2 on Android (#2963)

Summary:
I'm trying to setup a simple perf test when running llama2 on Android.  It's naively sent a prompt and record the TPS.  Open for comment about the test here before setting this up on CI.

### Testing

Copy the exported model and the tokenizer as usual, then cd to the app and run `./gradlew :app:connectAndroidTest`.  The test will fail if the model is failed to load or if the TPS is lower than 7 as measure by https://github.com/pytorch/executorch/tree/main/examples/models/llama2

Pull Request resolved: https://github.com/pytorch/executorch/pull/2963

Reviewed By: kirklandsign

Differential Revision: D55951637

Pulled By: huydhn

fbshipit-source-id: 34c189aefd7e31514fcf49103352ef3cf8e5b2c9