Don't prematurely delete temporary files

When verification and encryption is used, the temporary file was deleted
prematurely.

Also, on next boot-up, scan the directory we use for temporary files to
clean them up if the user rebooted in the middle of installation.

Bug: 7132197
Change-Id: Ic9e8aab9a664ca00d9ac16f72e53fc43d78d21bc