Set listener properties for UMS mount/unmount

Some kernels require the usb functions be defined before UMS mounting
of storage partitions will work. This sets the sys.storage.ums_enabled
property upon mount/unmount which can be listened for in init.rc.

Example routines to include in init.rc:

on property:sys.storage.ums_enabled=1
    write /sys/class/android_usb/android0/enable 0
    write /sys/class/android_usb/android0/functions adb,mass_storage
    write /sys/class/android_usb/android0/enable 1

on property:sys.storage.ums_enabled=0
    write /sys/class/android_usb/android0/enable 0
    write /sys/class/android_usb/android0/functions adb
    write /sys/class/android_usb/android0/enable ${service.adb.root}

Change-Id: I8a8ad4b392abbaa173d402714bcb4049ba84ccca