[8/N] WindowDecor refactor: Move WindowDecor#addWindow to HandleMenu

Handle menu is the only one that uses WindowDecor#addWindow, so this
change moves the method to better encapsulate menu logic.

Bug: 409648813
Flag: com.android.window.flags.enable_window_decoration_refactor
Test: m
Change-Id: I588610fa61e23a254d9523ed75e5c3df6be5a9fc