Add HAL definition for graphics buffer allocator

It differs from gralloc1.h in that

 - buffer descriptors are created from a struct, BufferDescriptorInfo, to
   reduce round trips
 - testAllocate is a function of its own
 - buffer allocation and export are two different steps
 - reference counting and buffer mapping are moved to gralloc-mapper
 - BAD_HANDLE is renamed to BAD_BUFFER
 - GRALLOC1_CONSUMER_USAGE_FOREIGN_BUFFERS is removed
 - CPU_{READ,WRITE}_OFTEN no longer implies CPU_{READ,WRITE}

Test: make android.hardware.graphics.allocator@2.0

Change-Id: Ibe9367d5b1701c0e1009da829f27fed0f7d98828