Replace compat UI related z values with constants in TaskConstants

Introduce a new constant in TaskConstants for compat UI components,
unifying previous individual constants.

Replace z-values in compatui window manager classes and in
Letterbox.

Fixes: 262403744
Test: atest WMShellUnitTests:LetterboxEduWindowManagerTest
Test: atest WMShellUnitTests:ReachabilityEduWindowManagerTest
Test: atest WMShellUnitTests:CompatUIWindowManagerTest
Test: atest WMShellUnitTests:RestartDialogLayoutTest
Test: atest LetterboxTest
Change-Id: If5e22014e38f22c0bd4371da459b52486c7f2d27