Allow native:vendor to vendor apks

Vendor apks should be able to reference native vendor libraries.
Vendor apks have worked around this by building without
LOCAL_SDK_VERSION, which then allowed to use all libs. However,
since vendor apks now needs to be built with SystemSDK, that
workaround can no longer be used.

Bug: 76398918
Test: BOARD_SYSTEMSDK_VERSION=P m -j
Change-Id: Idb13d5db71f4dfd542658483b6a24e7ece18ce26