Fixes https://github.com/flutter/flutter/issues/134566.

Prior to this fix, `ShutdownHooks` were run in the private helper
function `_exit()` defined in the `package:flutter_tools/runner.dart`
library. Independent of this, the tool had signal handling logic that
traps SIGINT and SIGTERM. However, these handlers called `exit()` from
`dart:io`, and didn't run these hooks.

This PR moves the `_exit()` private helper to
`package:flutter_tools/src/base/process.dart` and renames it to
`exitWithHooks()`, so that it can be called by the signal handlers in
`package:flutter_tools/src/base/signals.dart`.