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This delayes final step in lock screen apps initialization (observing
session state, thus preventing the state controller from reacting on
session being locked) until stylus input is detected (for one settings
UI for controlling lock screen apps is only shown for stylus enabled
devices).
Remove somewhat hacky solution where lock screen apps were disabled by
simply not reporting any app lock screen enabled.

BUG=765029

Change-Id: I214dbe09e95babdd4779268fa6abfa5c1615e80b
Reviewed-on: https://chromium-review.googlesource.com/693961
Commit-Queue: Toni Barzic <tbarzic@chromium.org>
Reviewed-by: Stefan Kuhne <skuhne@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#506904}