Add support for P+ environments

Originally-by: Alistair Strachan astrachan@google.com

With Android P, the GraphicBufferMapper ImportBuffer interface has changed, which breaks the current drm_hwcomposer master branch: https://android.googlesource.com/platform/frameworks/native/+/dbbe33b95336efa74e8bb4ebcf6cba50919aa247

Alistair has updated the AOSP/master branch of drm_hwcomposer to make it build: https://android.googlesource.com/platform/external/drm_hwcomposer/+/4f73630dcdab6604a3f4b3e7d59068633d923745%5E2..4f73630dcdab6604a3f4b3e7d59068633d923745/

But since we need to keep older users working, so I've forward ported and conditionalized the code so both new and old users can properly build.

Also includes fixes for the FIXME issues Alistair pointed out in the initial patch.