resourceloader: Fix broken skinStyles loop in #getModifiedTime

The way we interpret the 'skinStyles' array in #getModifiedTime
is broken. For comparision, look at the loop in #getStyleFiles
which we use for the creation of the actual stylesheets (which
works fine).

It mixed up the logic causing it to always end up with an empty
array. So it's not so much that it fails to detect the files
having changed after an update, it never included mtimes of these
files in the first place. If there'd be a module with only
skinStyles resources, it would have timestamp 0 (1970).

Bug: 62068
Change-Id: I2e772a13183e66e4bfbf95057ebfd7f5e0d817ec