Partial Screen Sharing: Task Switcher - Add method exposing session to callback

Adds a new method to MediaProjectionManager#Callback to inform when the
ContentRecordingSession was set.

This is needed for consumers that require more information about the recording
session, such as the content that is being recorded (screen vs task).

Bug: 286201261
Test: atest WmTests:WindowManagerServiceTests
Test: atest FrameworksServicesTests:MediaProjectionManagerServiceTest
Change-Id: Ie354d62dbc3896bd42079f2e5b5fb82cdb12706e