OOBE - Fix the code inclusion of display_manager.js

Remove the GN include directive of 'display_manager.js' in
'multi_step_behavior.js'. This is likely how DisplayManager
was being used in 'assistant_optin.js' and the directive was
moved there instead.

Also remove the GN include of 'display_manager_types.js' in
'display_manager.js' and move it to 'oobe.js'. This enables
these two files to be separated in order to be modulized for
the Polymer3 migration.

Bug: 1184731
Change-Id: I122690dcd879f685158686e12d03ee493165e560
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2993191
Commit-Queue: Renato Silva <rrsilva@google.com>
Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#896919}