Use $state_path/conversion for image_conversion_dir default

image_conversion_dir currently defaults to /tmp.  Using
$state_path/conversion seems to be a more appropriate default.

Closes-Bug: #1217552
Change-Id: Iaaa462449a257b1b48fc349399bf409d7301a1e6
Merge "Use $state_path/conversion for image_conversion_dir default"