graphics: add allocator HAL support library

Add a header-only library
android.hardware.graphics.allocator@2.0-hal that can be used by
implementations.  An imlpementation can

  class VendorHal : public AllocatorHal { ... };

  auto allocator = std::make_unique<Allocator>();
  allocator->init(std::make_unique<VendorHal>(...));

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

  class AlocatorHalExt : public AllocatorHal { ... };
  class VendorHal : public AllocatorHalExt { ... };
  class AllocatorExt : public AllocatorImpl<IAllocatorExt, AllocatorHalExt> { ... };

  auto allocator = std::make_unique<AllocatorExt>();
  allocator->init(std::make_unique<VendorHal>(...));

Test: builds
Change-Id: I7cb7a4888316b871e5c49d96524b1642fc708f2d