Fix NPE when assistant component name is null

Also moves user ID retrieval to UserTracker instead of binder call.

Test: Ensure device is provisioned and assistant invocation is enabled
Test: Add new user
Test: Perform assistant invocation
Test: Observe that no crash occurs
BUG: 236751191
FIX: 236751191
Change-Id: I3d3bb12cbfdcc62ec62ddc0d54889e71f2d85704