Rename vndk apex according to vndk version

VNDK APEX should be named after the version of VNDK libs.
For example, if vndk_version is 29, then the apex name should be
com.android.vndk.v29. If vndk_version is not set or is 'current', then
the platform vndk version is implied.

This is done with setting "apex_name" with proper name.

Bug: 139774701
Test: m (soong test added)
Change-Id: I918252f12ccd351886030fe9139a020d6cf1ff32