SurfaceTexture: fix a getTransformMatrix crash.

This change moves the computation of the transform matrix out of
getTransformMatrix and instead performs the computation when
updateTexImage gets called.  This is needed in order for
getTransformMatrix to succeed even if the buffers have been freed (e.g.
by changing the buffer count) because the computation depends upon the
properties of the current GraphicBuffer.

Change-Id: Ied541ab8747b7ad604f862717135f9a16a816be9
Bug: 4490420