Dont update thumbnails of freeform windows on focus change

Windows should not update their thumbnails upon a focus change since
it costs a lot of time to capture the content and the thumbnail would
be outdated short time later anyways since the window is still
visible anyways.

Bug: 23562904
Change-Id: If5d23a1058e83030770905827d682f649a579f7a
2 files changed