Do updateWorkspaceScreenOrder inside a single transaction.

The workspacescreens table is updated in LauncherModel.updateWorkspaceScreenOrder
and that the operation to remove all screens, then reinsert the new list of
screens in not inside a single transaction, so if the app is updating or
crashes between ContentResolver.delete and ContentResolver.bulkInsert then
the data will be lost. This CL makes it all happen inside 1 transaction.

Bug: 12523285
Change-Id: I409dbc9f48fa9c8bd4bf3b1453204a4daac1689a