libsnapshot: Fix vts_libsnapshot_test for GRF with Vendor on S

If the vendor partition is on S and system partition is on T,
certain tests in vts_libsnapshot_test used to fail. This is primarily
because of inconsistent check between daemon and vts test.

vts test checks the userspace.snapshots.enabled property which is true on T
but never checks if the underlying vendor partition is on S. Hence,
vts test will enable userspace snapshots. However, daemon checks
the vendor partition and disables userspace snapshots thereby
leading to inconsistency.

This is only a problem on vts tests. The underlying OTA on devices
works fine as we have the vendor partition check.

Bug: 236311008
Test: vts_libsnapshot_test on S vendor and T system
      vts_libsnapshot_test on T vendor and T system
Ignore-AOSP-First: cherry-pick from aosp
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: Iad4f299bd2e07c9c01f5fbee6a20e2f01bf1778a