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Depending how early EXO is initialize the Screen object may have already
initialized its display state. When CastWMHelper adds its display
observer it will not get a OnDisplayAdded() event by default. Query the
list and initialize the observer to ensure state is correct.

Bug: b/138247323
Test: Manually on device to verify OnDisplayAdded() is called before EXO
clients call to ask for display info.

Change-Id: I473185fda846e64907eebb808141d51f281fbdde
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1720993
Auto-Submit: Daniel Nicoara <dnicoara@chromium.org>
Reviewed-by: Michael Spang <spang@chromium.org>
Commit-Queue: Daniel Nicoara <dnicoara@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681384}