Use PlatformVndkVersion to find ABI dump by default

This commit changes "current" to ctx.DeviceConfig.PlatformVndkVersion().
This change make it possible to place ABI dumps in
prebuilts/abi-dumps/vndk/${version} instead of
prebuilts/abi-dumps/vndk/current.

If BOARD_VNDK_VERSION is not specified or is equal to "current",
PLATFORM_VNDK_VERSION (either PLATFORM_SDK_VERSION or
PLATFORM_VERSION_CODENAME) 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.

In the future, if the system image and vendor images are separately
built in different source trees, BOARD_VNDK_VERSION will be set to other
values and we should respect that value.

Test: Create an ABI dump under
prebuilts/abi-dumps/vndk/${PLATFORM_VNDK_VERSION}, make some ABI
breakages, and see whether the checks are working.

Bug: 76036094
Change-Id: I6242e0c71ebd9acd5c4a3497c67539cb3e6663f3