anv/android: support import/export of AHardwareBuffer objects

anv/android: support import/export of AHardwareBuffer objects

v2: add support for non-image buffers (AHARDWAREBUFFER_FORMAT_BLOB)
v3: properly handle usage bits when creating from image
v4: refactor, code cleanup (Jason)
v5: rebase to b43f955037c changes,
    initialize bo flags as ANV_BO_EXTERNAL (Lionel)
v6: add assert that anv_bo_cache_import succeeds, add comment
    about multi-bo support to clarify current implementation (Lionel)

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>