libui: harden GraphicBufferMapper::importBuffer

Add support for validateBufferSize and getTransportSize from IMapper
2.1.  Update GraphicBufferMapper::importBuffer to validate buffer
size, and update GraphicBuffer::flatten to use the handle transport
size.

This fixes two issues with GraphicBuffer.  Pointers returned by
lock/lockYCbCr can now be accessed without potential OOB.  flatten
no longer includes process-local runtime data.

Bug: 62535446
Bug: 62084097
Bug: 32587089
Test: manual
Change-Id: Ice13af26b84f25e43089637e9d67e3ad820e22ed