Rename AppsModelBuilder -> ExtensionAppModelBuilder, make safe.

Currently AppsModelBuilder assumes all items are ExtensionAppItems.
This will no longer be true as soon as we add AppListFolderItems.
We need to make AppsModelBuilder only apply to extension items.

By design we no longer wish to sync app list ordering with the extension
ordering used by the NTP. This CL will only use the extension ordering
for initial sorting, and as a result app list order changes will not be
preserved until we start saving / syncing the app list order separately,
crbug.com/305024

BUG=303839
For ash/shell/app_list.cc

R=jennyz@chromium.org, koz@chromium.org, xiyuan@chromium.org
TBR=jamescook@chromium.org

Review URL: https://codereview.chromium.org/26328004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227808 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227808