Disable letterbox education during app compat flicker tests

Fixes: 311868450
Test: adb root
&& adb shell rm /data/user_de/0/com.android.systemui/shared_prefs/has_seen_letterbox_education.xml
&& adb reboot
&& atest WMShellFlickerTestsOther:OpenAppInSizeCompatModeTest
&& atest WMShellFlickerTestsOther:OpenTransparentActivityTest
&& atest WMShellFlickerTestsOther:QuickSwitchLauncherToLetterboxAppTest
&& atest WMShellFlickerTestsOther:RepositionFixedPortraitAppTest
&& atest WMShellFlickerTestsOther:RestartAppInSizeCompatModeTest
&& atest WMShellFlickerTestsOther:RotateImmersiveAppInFullscreenTest

Change-Id: Ie77a0a906f941aca1321d3f2d8cdcc6316be3d8e