Grouper: Add PRODUCT_MANUFACTURER to omni_grouper.mk

This correctly inserts ro.product.manufacturer=Asus in
build.prop instead of ro.product.manufacturer=unknown,
as it is now. This also puts Asus Nexus 7 instead of
the awkward Unknown Nexus 7 in Google Store.

Change-Id: I6adfc874ee745389f4bc2091f61751715a937345