SurfaceTextureClient: support for application buffer dimensions

Add a new API native_window_set_buffers_user_dimensions to allow native
applications to override the native window size for the default buffer size.
This has lower precedence than the existing
native_window_set_buffers_dimensions and allows the two to co-exist.

Change-Id: Ie73590e1c94ef0dadbce500bd0941dfabbcace3c