Exclude mount entries for vendor overlay

When testing adb-remount with overlayfs, vendor overlay may fail the
test because there are unexpected overlayfs uses by vendor overlay.
By excluding mount entries for vendor overlay, the test runs
regardless of vendor overlay mount.

Bug: 122231184
Test: run adb-remount-test.sh with vendor overlay mount
Change-Id: I795774b27d5e5f973792337813b67487a5d72e29