[Contextual Edu] Change Toast to Dialog

The default style toast is obscured by taskbar in overivew. Toast does not support changing of position or customizing view anymore after Android 11.
This CL changes the displayed component from Toast to Dialog and moves up the dialog with configured bottom margin.
It also faciliates future change to customize the dialog in b/369791926

Bug: 369800039
Test: ContextualEduUiCoordinatorTest
Flag: com.android.systemui.keyboard_touchpad_contextual_education

Change-Id: Idf1c5642d2561fb608824ffd3d1baed214fde828