Fix apex_vndk with TARGET_VNDK_USE_CORE_VARIANT

When TARGET_VNDK_USE_CORE_VARIANT is set, vndk apex should have only
- vndk sp libraries
- vndk core libraries which are marked as "must use vendor variant"

Previously, vndk sp libs are discarded when TARGET_VNDK_USE_CORE_VARIANT
is set.

Bug: 143374795
Bug: 139772411
Test: TARGET_VNDK_USE_CORE_VARIANT=true m com.android.vndk.current
      see if vndk sp lib(e.g. libunwindstakc) is included.

Change-Id: Iae218fff5c354eb874839a1f71be30633fb8a9ab