window: add API for native_window_set_buffers_user_dimensions

Add token and wrapper function for a new private function
native_window_set_buffers_user_dimensions which overrides the native window
size as the default buffer size.  This is intended for use primarily by
ANativeWindow_SetBuffersGeometry so that application-set buffer dimensions may
co-exist with pre-rotation in e.g. the GLES driver which calls
native_window_set_buffers_dimension.

Change-Id: I6c0fc334c070290cc15b04daae1484d0a28cd8ad