[Contextual Edu] Show dialog with correct theme in light and dark modes

Found an issue that the edu dialog is always in dark theme.
This change provides the correct theme for light and dark modes.

Bug: 369800039
Test: Manual, checked the color of dialog is right in both light and dark themes
Flag: com.android.systemui.keyboard_touchpad_contextual_education
Change-Id: Ifa313a4bb28716cca5b1f12c12fbcec6d7e6b955