[1/n] Disable reachability for thin letterboxing

Reachability provides no advantages for users in case of thin
letterboxing so it can be disabled along with the related
educations.

Fix: 334077350
Test: atest WmTests:LetterboxUiControllerTest
Test: atest WmTests:SizeCompatTests

Change-Id: Ie00084ab5e6be1f3ff0eed8c3551720940859984