[40/n] Use taskId as unique key for letterbox surfaces

Each task has a unique id which can be used as the key to identify
each set of letterbox surfaces in it. The displayId is not always available
in TaskInfo which makes difficult to use it as part of the letterbox
surfaces identifier when a TaskListener or TaskVanishedListener are used.

Flag: com.android.window.flags.app_compat_refactoring
Fix: 416217659
Test: atest WMShellUnitTests:SingleSurfaceLetterboxControllerTest
Test: atest WMShellUnitTests:MultiSurfaceLetterboxControllerTest
Test: atest WMShellUnitTests:LetterboxInputControllerTest

Change-Id: I56e6c88d54933e4d5da87968bf9a83a77cf7528b