Set __ANDROID_API__ for vendor binaries to vndk version.

When building vendor modules with BOARD_VNDK_VERSION=current, the
API of the vendor modules will be current PLATFORM_VNDK_VERSION.
__ANDROID_API_FUTURE__ will be used as before if the version is a
CODENAME.

If BOARD_VNDK_VERSION is not "current", that means the VNDK version
of the vendor modules is BOARD_VNDK_VERSION.

Bug: 74833244
Test: Build and check boot.
Change-Id: Iccc603f1b5ffa066e74ba1357f3fbbcc13c58f76