libs/usb4: Refactor UserId and tests

Add a newtype, UserId, in apis for better type safety. Also re-write
tests to reduce dependency on specific sleep durations (opting for
polling + a long timeout).

Bug: 433329064
Test: atest frameworks/base/libs/usb4/tests/pci_authorizer_test.rs
Flag: com.android.server.usb.flags.enable_usb4
Change-Id: Iabad66adf7c6e18d010fcf20c971a73ed5c2d8e9