Qualcomm AI Engine Direct - update build flow (#1447)

Summary:
Summary
- Retire deprecated method in QnnExecuTorchBackend.cpp
- Update build.sh
- Update build instrcutions in setup.md
- Add CMAKE_FIND_ROOT_PATH_BOTH to executorch-config.cmake for cross-compile
- Update cmake rules for examples/qualcomm

Pull Request resolved: https://github.com/pytorch/executorch/pull/1447

Reviewed By: dbort

Differential Revision: D52338915

Pulled By: larryliu0820

fbshipit-source-id: 6792421e4132bc114411b7ac95baccea12501132