SurfaceTexture: add getTransformMatrix tests.

This change adds a test verifying that with no transform set on the
SurfaceTextureClient, the SurfaceTexture will return an identity
transform matrix.  It also verifies this same effect in the presence of
an additional call to native_window_set_buffer_count just before the
call to getTransformMatrix.

Bug: 4490420
Change-Id: Ic5adfa29b5696cc2b451433834e3758ef20c5edd