adb-remount-test: Refactor test cleanup

* If adb remount calls for a reboot during cleanup, this means it's
  trying to recreate vendor overlay. Don't reboot in this case because
  it's pointless. Total test runtime reduced by one reboot.
* Since this entire script assumes /system & /vendor must exist and
  remountable, add them to the MOUNTS list unconditionally.
* Remove /system/hello & /vendor/hello test, as we can just loop over
  MOUNTS to check those.

Bug: 243116800
Test: adb-remount-test
Change-Id: I2360314c404ee247356146760314c91ba2795ff5