Floss: Explicitly use topstack runtime to spawn cleanup tasks

It is observed that in handling SIGTERM, the tokio::spawn could panic
because of no Tokio runtime at the time.

This explicitly uses topstack runtime to avoid the panic.

Bug: 359055353
Tag: #floss
Test: mmm packages/modules/Bluetooth
Test: rmmod btusb & modprobe multiple times to ensure no crash
Flag: EXEMPT, Floss-only changes
Change-Id: I7acd03030c49b46f7873c0fb6358490d13a032d1