exports LOCAL_SOONG_VNDK_VERSION for VNDK libs

For vndk_prebuilt_shared module, it is set by 'version' property.
For other vndk libs(e.g. cc_library with vndk.enabled), it is set as
PLATFORM_VNDK_VERSION.

Background:
To support "skip installing current VNDK".

You can get a system.img without current VNDK libs.
This may help when you want a smaller system.img given that the image
will be used with a specific version (not current) of vendor image.

Bug: 132140714
Test: m TARGET_SKIP_CURRENT_VNDK=true && see if current VNDK is not
installed

Change-Id: I1c603efc3e95fe8bdf870f2de91994077899fca7