libsnapshot: add snapshotctl

... which handles merge when boot completed. It also
dumps debug information when requested.

Bug: 135752105
Test: adb shell su 0 snapshotctl dump
Test: call snapshotctl merge at different stage of OTA:
  - before OTA (exit normally)
  - during OTA is applied (exit with error)
  - after OTA is applied but before reboot (exit with error)
  - after reboot (finish merge and exit normally)
  - Manually stop its execution during merge and run again
      (finish merge and exit normally)

Change-Id: Idcc6aea8d7bbeb9a1a288c966b8f5e14b3f6a3e7