WindowStateAnimation: Clear transformation clip rect.

Ensure the clip rect taken from transformations is cleared
when there is no transformation. Normally the failure to
clear the clip would not cause a problem, as the final clip
would be equal to the window size. However, in the event that
the window scale goes on to change (WindowState::m(H/V)Scale that is)
the clip will now be specified at the inappropriate scale (notice the
way the clip must be divided by H/VScale as SurfaceFlinger will apply
the surface transform to the clip).

Bug:24298694
Change-Id: I4548e8ecea8d66d4942e99823653a7b05f87cea0
(cherry picked from commit 2f0fe62c)