libui: add support for android.hardware.graphics

Add Gralloc2::Allocator and Gralloc2::Mapper as wrappers to
android.hardware.graphics.allocator@2.0 and
android.hardware.graphics.mapper@2.0 respectively.  Prefer
Gralloc2::{Allocator,Mapper} in
GraphicBufferAllocator/GraphicBufferMapper.

The new path has these differences

 - it does not support gralloc0
 - it does not dup-and-close fence fds to and from HAL
   (not sure why we did that)
 - release implies native_handle_close and native_handle_delete
   (same as in gralloc1?)

When all interesting targets have android.hardware.graphics
implementations, the old path will be removed.  That is why the new path
is added in the least intrusive way, as in:

  if (new-path-valid) {
    new-path;
  } else {
    old-path;
  }

despite the ugliness.

Test: booted to launcher, tested with YouTube and some games
Change-Id: Idabb6c62e73c96a59736b497c6d6d1366095e7e3