graphics: add IMapper to complement IAllocator

IMapper is loaded in-process and is defined in C++ instead of HIDL.  To
guarantee binary compability, the interface is a struct of function
pointers instead of a class of pure virtual member functions.  The
functions pointed to by the function pointers must also have C-linkage and
have all of their parameters PODs.

Implementations are expected to be installed to

  /oem/lib/hw/android.hardware.graphics.mapper.hallib.so
  /vendor/lib/hw/android.hardware.graphics.mapper.hallib.so
  /system/lib/hw/android.hardware.graphics.mapper.hallib.so
  (lib64 instead of lib for 64-bit implementations)

The loader will look for the symbol "HALLIB_FETCH_Interface" and use it to
fetch the interface.

Test: make android.hardware.graphics.mapper@2.0
Change-Id: I3a2c7a6ce18a90adda6df2d89559c957a122fdf0