graphics: add mapper HAL support library

Add a header-only support library
android.hardware.graphics.mapper@2.0-hal that can be used by
implementations.  There are two classes in the support library.
MapperHal is an abstract class to be implemented by implementations.
Mapper is an implementation of HIDL IMapper interface on top of
MapperHal.

An implementation can

  class VendorHal : public MapperHal { ... };

  auto mapper = std::make_unique<Mapper>();
  mapper->init(std::make_unique<VendorHal>(...));

Or, if vendor extensions are to be added to the IMapper,

  class MapperHalExt : public MapperHal { ... };
  class VendorHal : public MapperHalExt { ... };
  class MapperExt : public MapperImpl<IMapperExt, MapperHalExt> { ... };

  auto mapper = std::make_unique<MapperExt>();
  mapper->init(std::make_unique<VendorHal>(...));

Test: builds
Change-Id: Ib23c1f5977744f7e116bb93db53e882e2dad7ce3