Fix NPE in WMS

In RootWindowContainer#getTopFocusedDisplayContent(), there is a chance
which the external display is disconected but mTopFocusedDisplayId is
not yet updated, which cause NPE when trying to use DisplayContent field
returned by RootWindowContainer#getTopFocusedDisplayContent().

Reroute to default displayContent when top focused displayContent is
null.

fix: 124773022
Test: atest WmTests

Change-Id: I8c59fd4f43ac17a44f643e75829e546487a8d4ec