Add missing dimen to res/values/dimens.xml.

overview_panel_list_padding was only
defined in values-sw320dp. If you actually switch to a resolution that
resolves to this density bucket, Trebuchet will crash because there is
no corresponding resource by that name in res/values/dimens.xml.

Also, remove unused resource overview_panel_side_padding which has the
same issue.

Change-Id: I2174110f5a6db4816a764704b1f7b71890ff3e66