ANDROID: GKI: add android_vendor.h

This header file is to be used for various macros to help add
vendor specific fields to kernel data structures to enable
value-added features implemented in vendor modules.

They are to be used before the freeze (to anticipate places where
vendor-specific data is required).

Bug: 156285741
Signed-off-by: Todd Kjos <tkjos@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: Ic23b2760ece93facdc51cc35718a4154aba810f9