vndk-def: Add gralloc-mapper to sp-hals.

This commit fixes the name for gralloc-mapper so that
vndk_definition_tool.py can correctly find gralloc-mapper
implementation.

Test: ./vndk_definition_tool.py sp-hal --system ... --vendor ..

Change-Id: I6ecf45ca6fdb107ca1854f51d620888d5eeaed5b