Parse the list of supported vndk snapshot versions

This allows the target, ld.config.$VER.txt, to be defined for all
$VERs for which a VNDK snapshot exists under /prebuilts/vndk/v$VER.
With this fix,
1) supported_vndk_snaphsot_versions do not need to
be manually updated everytime a new VNDK snapshot is added and,
2) ld.config.$VER.txt will not be ill-defined in a tree that does
not have the required dependency files under /prebuilts/vndk/v$VER.

Test: m -j ld.config.28.txt
Bug: 74658756
Change-Id: Idb056c21412d4cb7c7a7cb3c247b1d82a4a759ff