android9: Probe for schedtune nesting before disabling it

Some kernels are now exposing the nested schedtune functionality, like the Pixel 3a, Fairphone 4 & JingPad. This allows us to make use of schedtune cgroup boosting on some patched devices.

Necessary kernel patch: https://github.com/fredldotme/android_kernel_google_bonito/commit/6452429d1f1dbdc9e1e310e82cc18664f2d1b56d

To accommodate for devices without such functionality attempt to probe the ability to nest schedtune cgroups inside each other and, if not possible, bail out and unmount the cgroup controller as before.