remount: If checkpoint is in progress, wait for it to complete

Instead of having the user to retry the `remount` command until success
(manual poll). Just block the remount command until checkpoint is
complete, and then continue.

Bug: 252989722
Test: adb remount on a cold device, remount command would block for a
  few dozens of seconds and then continue.
Test: adb remount on a warm device, remount command would process
  immediately. Do not show "force end checkpointing" instructions.
Change-Id: I65f3a5ade4c9538a55892345c75b45ac3a1755fd