Porting over ET MultiModal Demo App (#4455)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4455

Adding ET demo app for multimodal support. This is the first diff that supports Llama3.

This includes major changes to the existing Llama Demo app with having the following features:
1. Llama3 support
2. Settings activity
3. UI/UX improvements to the mainactivity
4. Ability to add multi-images in prep for multimodal support.
5. Metrics

Note: You'll need to build the `executorch-llama.aar` and have it placed in the `app/libs` folder.

Reviewed By: kirklandsign

Differential Revision: D60416605

fbshipit-source-id: 262329c30e1ec28c3905da5c040dc661307f8666