Add hidden API to get vendor genfs version

As genfs_contexts depends on vendors, users should be able to read the
genfs versions of vendors to implement fallback mechanism for new
features. This adds a new hidden API reading the version.

To prevent from diverging, the hidden API uses native libraries
(libgenfslabelsversion).

Bug: 378805380
Test: create a bundled app, call SELinux.getGenfsLabelsVersion
Change-Id: I8993f8dca9a7e9e6aff835e94ba6e5bc87612e1d