Added API to set assistants UID.

Added API to set the UIDs of the services that can be used as assistants.
Also modified code that uses the previous APIs for setting the unique
assistant UID and its corresponding hotword service. The new API can set
multiple UIDs all which will be treated the same for the purposes of
proviving assistant services.

Bug: 189312611
Test: m -j, and run assistant
Change-Id: Ieb28b7141206b58ae1d5d68580c08f4c733560b6