adbd_auth: return auth id when requesting a prompt.

When multiple auth requests come in (e.g. if someone connects over TCP
and USB, or if we send a USB request, and then kill adb and try again),
we need to know the ID assigned to the request to disambiguate.

Bug: http://b/159061108
Test: manual
Change-Id: I68cf2335c5958decf29e58ed9132735577a7e074