Bring abi references directory logic in sync with soong.

If BOARD_VNDK_VERSION is not specified or is equal to "current",
PLATFORM_VNDK_VERSION will be chosen. Currently, BOARD_VNDK_VERSION is always
equal to "current" because both system and vendor images are built in a single
source tree.

Bug: 64267858

Bug: 76036094

Test: utils/create_reference_dumps.py

Change-Id: I3bc76fc0029e9656881e2d8d1b2b80272120f6e6