graphics: clarify importBuffer and passthrough HALs

A buffer handle recieved from a HAL is by definition raw and needs
to be imported.  But because of passthrough HALs, such a raw handle
may have been imported already.  Explicitly specify that an
implementation must accept such a raw handle.

Bug: 37540361
Test: boots on angler, ryu and marlin
Change-Id: I5ecf526e59b27cc4a8f7f5d5ec27477da0946ece
4 files changed