Experimental Bubbles client controller

Certain privileged applications that are tightly integrated with the
system and SysUI need to be able to create and control multitasking
features to achieve a well-integrated and polished user experience.

The change adds a hidden API for interacting with the multi-tasking
controller that allows applications to request actions and
configurations related to system multitasking features like Bubbles
implemented in WM Shell through the client interface.

The calls from the client first come to a controller class in WM Core
(MultitaskingController), where the policy layer verifies the
eligibility of the requests and then relays them to the registered
delegate in WM Shell (BubbleMultitaskingDelegate).

The change introduces a new privileged permission required to
interact with the new APIs.

The new APIs are added in WM Extensions in BubbleContainerManager for
the application clients.

Bug: 407149510
Test: Manual, using DroidNavigator app
Flag: com.android.window.flags.enable_experimental_bubbles_controller
Change-Id: Idc2523f7de0773b6c97171903c9772b78d1d957f