Deprecate Vendor Overlay when VNDK is deprecated

Vendor overlay was introduced to enable system-only update with small
changes on the vendor, but this is no longer required with same reason
of VNDK deprecation - more frequent Vendor update than expected, and
makes system-vendor interface unstable. This change adds more comments
to explain that Vendor overlay will be deprecated along with VNDK, and
skip test if VNDK is deprecated.

Bug: 307925435
Bug: 307902290
Test: fs_mgr_vendor_overlay_test passed with CF trunk-staging device
Change-Id: I7b9359a5754e8740e749c48f6265a1b0f92f13af