Deal with images whose size is smalled than default

When an image is smaller than the default size, the placeholder 'default'
value will be the image's actual size. This means that if a user selects
default thumb size for the image, it won't be changed unless the original
size is bigger than the default. Also adjusted so that default size limits
width only, and doesn't act as a bounding box on size change.

Bug: 62293
Change-Id: I8341776794843ff63a93bedeadf5921851c57a26
Merge "Deal with images whose size is smalled than default"