... and use to remove zx_thread_set_priority() call from usb-xhci.

devhost connects to scheduler.ProfileProvider, and allows drivers to
request a profile. This is an intermediate state until we decide if/how
we want to allow drivers to consume fidl services.

ZX-3703 #comment [syscalls][usb][xhci] Introduce scheduler profile to devhost
ZX-2914 #comment [syscalls][usb][xhci] Introduce scheduler profile to devhost

Test: CQ, manual instrumentation/logging
Change-Id: I23359f2062b77cf84e3bccef72682668817618ad