Don't set Allocation mSize when we have no Type.

Bug: 10667740

A3D creates Allocations before knowing the underlying Type (and thus size).
Moving the mSize calculation to be conditional for concrete Allocations
eliminates the bug. In the future, we could let A3D dynamically update the
Allocation size if it is necessary to improve heap behavior.

Change-Id: I520246806b6ead0387a1a41372dade1a6e7c2271