Rename executorch-llama.aar to executorch.aar (#6313)

Summary:
Let's use a uniform name. It's easier to maintain and less error prone.

It won't cause BC issue. All renaming are happening within this repo, this version. For user apps, after they download or build the AAR, they can rename in their build.gradle.kts.

Pull Request resolved: https://github.com/pytorch/executorch/pull/6313

Test Plan:
1. `sh examples/demo-apps/android/LlamaDemo/download_prebuilt_lib.sh` run twice!
2. `sh build/build_android_llm_demo.sh`
3. `sh examples/demo-apps/android/LlamaDemo/setup.sh`
4. `sh examples/demo-apps/android/LlamaDemo/setup-with-qnn.sh`

Reviewed By: cmodi-meta

Differential Revision: D64501756

Pulled By: kirklandsign

fbshipit-source-id: 1ea7c58478f467ebc53cdaaa355cb117e0bfea08