Change the return type to null for the

synchronous USD publish/subscribe methods.

USD may be offloaded to a component
outside of supplicant, so we may not
be able to guarantee that the ID will
be available immediately.

Since the ID is also returned in the
callback, we can retrieve it from
there instead.

Bug: 340878198
Test: m
Change-Id: I69fc954ba028f33f74b226d1c4597700551ec21f