Exclude recent tasks within an ActivityView

singleTaskInstance is set by ActivityView to limit
the display to a single task. This also itentifies
the VirtualDisplay associated with an ActivityView.

Since an Activity within ActivityView cannot be
brought to front via recents, they are suppressed.

Test: atest WmTests:RecentTasksTest
Bug: 123587368
Bug: 126185105
Change-Id: I6cf913da793d2fab919a0d7ec88e7965fa50a257