Choose SAbi files with module's vndk version

No matter what BOARD_VNDK_VERSION is set to, VNDKs should be regarded as
PLATFORM_VNDK_VERSION. This fixes an abidiff error due to the version
mismatch.

Bug: 157106227
Test: manual test with BOARD_VNDK_VERSION
Change-Id: I0d262b8ee5ac6f40fea7c2fb18322531434c0b4b