drm_hwcomposer: Andorid.mk : Mark libdrmhwc_utils as vendor module

drm_hwcomposer: Andorid.mk : Mark libdrmhwc_utils as vendor module

To allow drm_hwcomposer to build with Treble, set
the libdrmhwc_utils library as a vendor module.

Cc: Marissa Wall <marissaw@google.com>
Cc: Sean Paul <seanpaul@google.com>
Cc: Dmitry Shmidt <dimitrysh@google.com>
Cc: Robert Foss <robert.foss@collabora.com>
Cc: Matt Szczesiak <matt.szczesiak@arm.com>
Cc: Liviu Dudau <Liviu.Dudau@arm.com>
Cc: David Hanna <david.hanna11@gmail.com>
Cc: Rob Herring <rob.herring@linaro.org>
Cc: Alexandru-Cosmin Gheorghe <Alexandru-Cosmin.Gheorghe@arm.com>
Cc: Alistair Strachan <astrachan@google.com>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
[jstultz: commit message tweaks]
Signed-off-by: John Stultz <john.stultz@linaro.org>