vold: implement resetCheckpoint

It will be used by userspace reboot to reset checkpoint state, to make
sure that when /data is remounted, it will be remounted in checkpointing
if a checkpoint was requested beforee userspace reboot.

Test: /system/bin/vdc startCheckpoint 1
      /system/bin/vdc needsCheckpoint (returns 0)
      /system/bin/vdc resetCheckpoint
      /system/bin/vdc needsCheckpoint (returns 1)

Bug: 135984674
Change-Id: Ia29238686289b4eed93e2fb936a8b3d894b94dc9