[MediaProjection] Do not start layer mirroring without a surface

An app may pass a null surface to MediaProjection#createVirtualDisplay,
and later set the surface on the VirtualDisplay. The display's state
will change from off to on when the surface is set.

When determining if layer mirroring should be started/updated in
DisplayContent, take the presence of the surface/display state
into account, to prevent the app from crashing.

Bug: 197863746
Test: atest WmTests:DisplayContentTests
Change-Id: I01af07fff681aab6fc8235c861375eb79118cbfe