gralloc: add reserved region

This patch adds a reserved region of shared memory to every gralloc
native_handle_t. The reserved region of memory will allow future
versions of Android to add new buffer features without requiring
gralloc upgrades.

Bug: 145232031
Test: VtsHalGraphicsMapperV4_0TargetTest

Change-Id: I1bdfec0a6ef85097096035d3d38e5c9e7fcc32f2