Update reachability UI styles to be decendants of Theme.AppCompat

Letterbox reachability uses AppCompatTextView's which requires its
themes to be descendants of Theme.AppCompat or Theme.AppCompat itself and produces a warning if else.

To prevent this warning from being triggered, set the reachability UI
styles to be children of Theme.AppCompat. Also update the reachability UI layout to declare styles using 'android:theme' instead of 'style' so ThemeUtils theme check can locate them.

Remove 'android:visibility="invisible" from style and apply to components individually and remove 'android:contentDescription' as it is not needed.

Fix: 281804869
Test: atest WMShellUnitTests:ReachabilityEduLayoutTest
atest WMShellUnitTests:LetterboxEduWindowManagerTest
atest WMShellUnitTests:ReachabilityEduWindowManagerTest

Change-Id: I3b62f1c5ccaa748326030b8c2812fb5fe7e5dfe5