Skip home activity in removeUnreachableHiddenTasks

For devices that support multiple displays, when connecting to a new
display, a new home activity will be added under the new display. It
triggers RecentTasks#removeUnreachableHiddenTasks() to clean up the
unreachable hidden tasks, which may also remove the home activity of the
old display(eg. when the task on the old display is in fullscreen, the
home activity is an unreachable hidden task and therefore, gets
removed). This is not a desired behavior because each display is
expected to have its own home activity.
This change adds a check to avoid removing the home activities in the
removeUnreachableHiddenTasks() method.

Bug: 274862596
Test: Manual test, make sure that connecting an external display doesn't
remove the original home activity.

Change-Id: I1b7e8c9706d8b5ab53766779aec63141ca0669ed