Send taskInfo instead of root task info in onBackPressedOnTaskRoot callback

Previously root task info was sent in onBackPressedOnTaskRoot callback. That information was redundant as root task info is already available while registered the task listener. This CL send the task info for which back event is received.

Flag: EXEMPT minor change

Bug: 425748492
Test: atest com.android.server.wm.WindowOrganizerTests
Test: Manual - Checking if the backevent received.
Change-Id: Ic9291e3d780a4d57a86eb8c193bc05141f1bdec4