fs_mgr: adb-remount-test.sh filter out ramdumpfs administrative mount

Causes flakes as the ramdumpfs may temporarily be applied
to check and/or retrieve content.

Signed-off-by: Mark Salyzyn <salyzyn@google.com>
Bug: 161454607
Test: no more flakes
Change-Id: I8e4b6a808ab81ec5b4f760a810b9b651a0b329d0