On low power turn off rather than rebooting to allow device to charge

On low power turn off rather than rebooting to allow device to charge

The code was using encrypted_upto == 0 as an indicator that encryption
has succeeded. This meant that if no encryption happened, we would reboot
continually.

We now set encrypted_upto to fs_size when encryption is complete.

Also don't start to encrypt unless we are at 10% power. Stop when we
get to 5% power. This should lead to partial encryptions only very
rarely.

Bug: 15513202
Change-Id: I6214d78579d1fbbe2f63ee8862473d86a89d29b3