adb-remount-test: Check override_creds only if overlayfs is used

Instead of probing the kernel to see if overlayfs is supported, just
check `df` after disable-verity.
If after disable-verity and overlays were mounted, then check that
override_creds patches are applied.

Bug: 243116800
Test: adb-remount-test
Change-Id: Icb1363278536a8177836263882b1a8a0d9f246c9