Allow keyguard dialogs to show on secondary displays.

When the keyguard or a dream is visible, we usually block content
from showing on secondary displays by mirroring the primary display
contents to them.  However, the keyguard may wish to show a
presentation on a secondary display in which case we should not
mirror.

This change adds an exemption for keyguard dialogs when the full
screen is obscured.  The keyguard can then create presentations with
the TYPE_KEYGUARD_DIALOG window type in order to show special
content on other displays selectively.

The old code used to cause all secondary displays to mirror, which
isn't quite what we want.

Bug: 11257292
Change-Id: I55429002b2233ae25fe80db149636d6f41f2a112