remount: Use MyLogger class and sundry improvements

* Use the MyLogger class from set-verity-state.cpp and eliminate the
  |verbose| global variable.
* Support specifying reboot() reason. Log error if reboot() failed to
  reboot.
* Move precondition checks after all |argv| are processed. This way (-h)
  help message can be shown w/o root user.
* Log "remount [succeeded|failed]" message before auto-reboot. Give
  users a clear succeeded/failed message.

Bug: 241688845
Test: adb-remount-test
Change-Id: If45de2eba0d532632de43b19c797ffdeea90cd6d