[60/n] Improve letterbox surfaces bounds calculation

The caluclation of taskBounds and letterboxBounds used to define
letterbox surfaces bounds needs to take offsets into account.
This allow to fix the problem related to Tasks that don't have
(0,0) as position.

Flag: EXEMPT Refactoring
Bug: 423072242
Bug: 377875664
Test: atest WMShellUnitTests:TaskInfoLetterboxLifecycleEventFactoryTest

Change-Id: Icbffcc537b7aa8800fd8f968a1628c8c37eb5a60