[2/n] Adds reachability education for double-tap

Double-tap education which explains to the
users how double-tap works for repositioning an app in
letterbox or size compat mode.

The education appears the very first time an app is launched
by a user and every time the user double-tap to reposition
the app.

Fixes: 255943773
Test: atest WMShellUnitTests:ReachabilityEduLayoutTest
      atest WMShellUnitTests:LetterboxEduWindowManagerTest
      atest WMShellUnitTests:ReachabilityEduWindowManagerTest

Change-Id: I583a2ca8e552885b3578eaa4e344d89a67143b57