Make size/checksum immutable for active images

Once an image becomes 'active' it should only be possible for an
administrator (and not a regular user) to change the size and checksum
values.

Addresses bug 1209148.

Change-Id: Ic65afbca4a4161dfa77ce0a296a1bd72d96902ca
Merge "Make size/checksum immutable for active images"