Prompt for confirmation before reboot if installation fails.

If previous installation fails, menu item 'Reboot system now'
and 'Power off' now prompts for confirmation from the user.

Known issues:
- If the sideload is interrupted, it'll still boot into normal
Android in the next cycle.
- If 'Enter fastbootd' is chosen, and then 'Enter recovery', such
prompt do not show up.

Test: manual
Fixes: 142892891

Change-Id: I929b80e0520bd3b9f56d88a4b2203fcdd8d7b013