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This CL supports showing toasts on lock screen.

If the device is locked and the toast can be in the lock screen,
ToastOverlay appears in kShellWindowId_LockSystemModalContainer
instead of kShellWindowId_SystemModalContainer.

kShellWindowId_LockSystemModalContainer can be used only during
the lock screen is visible, so we switched the container when the
lock status is changed.

Bug: b/79951342
Test: ash_unittests --gtest_filter="ToastManagerTest.*" passes

Change-Id: I40d62e21b437686fcfc4b119a0c9954207f3533d
Reviewed-on: https://chromium-review.googlesource.com/1154783
Commit-Queue: Yoshiki Iguchi <yoshiki@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579866}