Add vndservicemanager to default builds.

vndservicemanager will be used for AIDL/binder
communication between processes running from
the /vendor partition. While technically only
required on full-treble devices, it is useful
to have on Android devices by default, since it
allows vendors to distribute a single binary talking
to vndservicemanager, instead of having to maintain
two versions that talk to different servicemanagers.

On devices that don't have a new kernel prebuilt with
/dev/vndbinder yet, vndservicemanager will simply
block indefinitely.

Also add new vndservice_contexts target, that
defines the service contexts for vendor services.

Bug: 36052864
Test: Marlin boots with and without /dev/vndbinder
Change-Id: I7334b5cb357dc6d4c5de8632e835dd892cd2f174