Exclude WindowState from transition targets

Otherwise it may look weird to check whether the WindowState can
promote to ActivityRecord. The case happens when transferring
splash screen window, which will collect the starting window.
And usually the window level animation is done by someone other
than shell.

Bug: 194112093
Test: atest SurfaceControlTest
Change-Id: I11868342b9a76a0561718f0e549b412eb4282f6a