swapart: don't use busybox swapon/swapoff builtin

The hybrid boot/recovery image means that certain commands are
utilized by /sbin/busybox. The busybox swapon builtin does not
support priorities, which breaks the swapart script. Fix this by
specifically referring to /system/bin/swapon, /system/bin/swapoff.

Change-Id: I55f675220e540bf180ed0ee8fd55b12d2833847b