Refactor HandleMenuActionButton to remove redundant LinearLayout

HandleMenuActionButton extended LinearLayout, but its constructor
inflated another LinearLayout as a child. This prevented direct
configuration of HandleMenuActionButton's padding in
desktop_mode_window_decor_handle_menu.xml, as the inner LinearLayout has
the default padding now.

This change uses <merge> in
desktop_mode_window_decor_handle_menu_action_button.xml, allowing direct
addition of child elements to HandleMenuActionButton and eliminating the
edundant inner LinearLayout.

Bug: 403072612
Flag: com.android.window.flags.enable_desktop_windowing_mode
Test: Manually check there is no user visible changes
Test: Manually check the talk back support does not regress b/398128081
Change-Id: I55ac41e56c644eb754e5edae18f19ec0296aace2