Fix education flows for thin letterbox

The education flow reached an inconsistent state when
the very first letterboxed app had thin letterboxing.

Now we ignore the reachability education in case of
thin letterboxed apps.

Flag: EXEMPT bugfix
Fix: 349117404
Test: atest WMShellUnitTests:CompatUIControllerTest
Test: Manual following steps on bug description
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:1d6f6bd2fa617af90457a7347d98558cb6d78a6a)
Merged-In: I7fb1ff2ee0540325218a379259c5277cff1367ad
Change-Id: I7fb1ff2ee0540325218a379259c5277cff1367ad