Fixed Task#getVisibility to take into account task hierarchy

Also, correct some uses of getRootHomeTask()

Fixes: 153086746
Bug: 154331729
Bug: 80414790
Test: atest WmTests:ActivityStackTests#testGetVisibility_MultiLevel
Change-Id: I8b9a38879a0b4e6c72686312b414319b8b086cd1
11 files changed