gralloc: Add variable reserved region size and default metadata checks

This change adds support for reserved regions larger than one
page, as well as indicating whether a metadata value is
default or has been explicitly set through set().
This fixes VTS failures for GetDataspace and
GetLargeReservedRegion.

CRs-Fixed: 2721357
Change-Id: I4bb428d429accb471a08bd44b88c3a565836a12c
3 files changed