Update Android and iOS demo app readme for Spinquant and QAT+LoRA model support (#6485)

Summary:
1. Added export command for spinquant and QAT+LoRA using prequantized model we are going to release at 10/24
2. Clean up on duplicated commands
3. Renaming the path in the export command to avoid confusion
4. Remove old information

Reviewed By: cmodi-meta

Differential Revision: D64784695

Co-authored-by: Chester Hu <chengpenghu@meta.com>