Set autofill id on virtual structures.

ViewStructure.newChild(virtualId) uses the AutoFillId of the parent,
which was not set anymore due do a previous refactoring, and was
causing this method to crash a custom view when
onProvideAutoFillVirtualStructure() was called on it.

Test: manual verification
Bug: 31001899

Change-Id: I602b421b0ec3a926ffdd52253d5b2498c4217d02