Swift SDK installation script improvements
### Motivation:
The SDK installation scripts had a few rough edges and didn't verify the
artefacts they downloaded.
### Modifications:
* Change naming to the more accurate "Swift SDK" where we can without
breaking downstream adopters.
* Separate Android NDK installation into its own script.
* Avoid using `cd` when setting up Android NDK.
* Verify checksums for SDKs and signatures for toolchains.
* Specify/control the SDK directory.
### Result:
* More accurate naming.
* More maintainable scripts.
* More secure downloads.
An example of this working
https://github.com/apple/swift-nio/actions/runs/18944235613/job/54090944716?pr=3429
