VNDK APEX doesn't use "apex_name" property

VNDK APEX has been using "apex_name" property to set "name" field in
apex_manifest. But setting apex_name had a side-effect of emitting
duplicate rules for symbol files for VNDK APEX and its override_apex.
(Please see the removed comments for more details about error)

Not using "apex_name" fixes the duplicate errors.

There're a few more vendor apexes with the same issue, which I will
handle in follow-ups.

Bug: 267581665
Test: m (soong test) && boot
Change-Id: I00f617cef8af4d21880d4e8b9c0b9ab9322ba15c