container_manager: Keep schedtune cgroup around in case nesting is supported

Certain devices with Ubunutu Touch make use of the schedtune cgroup boosting mechanism,
allowing for better performance system-wide with very little tweaking.
Some devices have trouble using this cgroup in a regular GNU/Linux setting because
their kernels are not carrying the necessary kernel patch for enabling nesting. [1]

To please both those that want Waydroid to stop dropping their performance as well as
those having unpatched kernels, probe the capability to nest schedtune cgroups.

[1]: https://gitlab.com/ubports/porting/reference-device-ports/android9/google-pixel-3a/android_kernel_google_bonito/-/commit/6452429d1f1dbdc9e1e310e82cc18664f2d1b56d