[Contextual Edu] Accessibility - Not change focus when ContextualEduDialog is shown

This toast-like dialog should be unobtrusive to user interaction:
1) should allow interaction with background elements
2) should not change the focus as user might be already focusing on another item

Bug: 372895069
Test: ContextualEduDialogTest
Flag: com.android.systemui.keyboard_touchpad_contextual_education
Change-Id: I2f0c341748cb651ab750fa738071c892d76b8e9f