Reorganize and clean up lock screen section

- Move the option for enabling widgets to the top of the widgets header,
  as it is 1) the most important option in that section and 2) most
  likely what the user's going to be looking for first.

- Use a simple "custom widgets" title for enabling widgets instead
  of the verbose "widget pager" term.

- Shorten and simplify overly long, hard to parse space-hogging
  summaries.

- Move "slider shortcuts" out of the widgets section.

Change-Id: Iac1b55a6b976c5645ca7f46e7fdf0821396e1100