Refactor fs_mgr_update_verity_state()

fs_mgr_update_verity_state() has two callers with generally different
intentions.  One caller loops through all entries in the default fstab
to set partition.<mount_point>.verified properties.  The other caller
is only interested in whether or a specific mount point has verity
enabled.

Given this, we refactor fs_mgr_update_verity_state() to
fs_mgr_get_verity_mount_point() which takes a single FstabEntry and
returns the mount point used for the dm-verity device or an empty
option if verity is not enabled on that mount point.

Test: adb-remount-test.sh test on blueline
Change-Id: Ic7dd8390509e95b2931b21e544c919a544138864