Enable WindowContext to set window and override type

There are some requests to use WindowContext as the notification
shade window container:
1. Leverage the WindowContext usages to its attached dialogs
2. Set the window type of dialog with minimum changes

The proposed solution are:
1. Set the notification shade window as the parent window
2. Set dialog as sub-window type

This CL adds two hidden APIs to fulfill the requirements.

Test: atest WindowContextTest
Flag: com.android.window.flags.enable_window_context_override_type
Bug: 398759994
Change-Id: I43b6d34ba24d338ea08ab67893a0e121cddcceb2