[hook_runner] Use `ProcessManager` and `FileSystem`

[flutter_tools unit tests](https://github.com/flutter/flutter/tree/master/packages/flutter_tools/test/commands.shard) are split into hermetic and permeable tests.

The hermetic tests mock the file system and process invocations.

* Mocking processes: https://pub.dev/packages/process
* Mocking file system: https://pub.dev/packages/file

The native_assets_builder does not use these abstractions, and will try to read/write the local file system and do process invocations.

This means we cannot write hermetic unit tests for the native assets feature, and that we have to always skip the check for native assets in the hermetic unit tests.