Move CommandQueue.addCallback later

In QSFragment, the callback was tied to the lifecycle of the fragment,
so it was only added after all the views had been assigned. As we don't
have a lifecycle here, we move it at the end of the onComponentCreated.

Also, just for protection, add null checks inside the callback.

Test: atest QSImplTest
Fixes: 303180152
Change-Id: I42130dd12e8c1cb3cf908147cda153bf4e6702d1