Enable the split button in app header menu on external display

The split button in app header menu on external display was disabled to unblock teamfood. Since the Launcher side animation is getting fixed, enable the button behind the bugfix flag com.android.window.flags.enable_non_default_display_split_bugfix

Test: manual
Bug: 432270801
Flag: com.android.window.flags.enable_non_default_display_split_bugfix

Change-Id: I43bab4979f4f661f8a3dd4601960ce9af131a271