libvendorsupport provides an API to replace vndk version

ro.vndk.version cannot be simply replaced with ro.board.api_level with
the vndk deprecation. Even with the latest system updates, devices may
still run on old vendor images that do not define ro.board.api_level,
but define ro.vndk.version.
To provide the replacement of ro.vndk.version, provide a platform API
AVendorSupport_getVendorApiLevel() to return the expected vendor
version.

Bug: 312311458
Bug: 312315580
Test: manual test for AVendorSupport_getVendorApiLevel()
Change-Id: Id7c04483956d95fd49414cebde41d7cc4d2fb1d1