Move !is_android to build_engine_artifacts declaration (#54006)

The `//flutter/build/archives:artifacts` target is used to build a zip
archive (artifacts.zip) of host tools such as flutter_tester, the Dart
kernel compiler, the impellerc shader compiler, and other tooling that
is bundled in debug-mode host builds.

This moves the `!is_android` to the definition of
`build_engine_artifacts`. This is required because of the way that we
produce 32-bit arm gen_snapshot for Android on Windows hosts, which
relies on the regular x64 host toolchain due to us having no 32-bit arm
toolchain for Windows. As such, `current_toolchain == host_toolchain` on
that platform.

```
build_engine_artifacts =
    flutter_build_engine_artifacts &&
    (current_toolchain == host_toolchain ||
     (is_linux && !is_chromeos && current_cpu != "arm") || is_mac || is_win)
```

On iOS builds, we don't have this issue since `current_toolchain` will
be one of:
* `//build/toolchain/mac:ios_clang_arm`
* `//build/toolchain/mac:ios_clang_arm_sim`
* `//build/toolchain/mac:ios_clang_x64_sim`

Whereas `host_toolchain` will be one of:
* `//build/toolchain/mac:clang_arm64`
* `//build/toolchain/mac:clang_x64`

This patch also adds documentation to clarify the purpose of this target
and where related artifacts are produced so that future readers don't
need to do a deep dive into our build plumbing to figure this out.

While the target itself is primarily intended for producing host
binaries, one target binary (gen_snapshot) is bundled into the same
archive bundle as the host tools. This should be refactored such that
just like iOS and Android, they are bundled into their own
target-platform-specific archive, and the tool code accordingly updated
to pull these down into the appropriate cache directory.

As a side-note, on macOS we do rely on this archive target for the host
tools, but the bundled gen_snapshot is unused -- instead, one produced
by the //flutter/sky/tools/create_macos_gen_snapshots.py script used.
This should be fixe in a followup patch.

Related: https://github.com/flutter/flutter/issues/38935

Identified while trying to resolve:
Issue: https://github.com/flutter/flutter/issues/101138
Issue: https://github.com/flutter/flutter/issues/69157

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