Visual updates to summary onboarding UI

- adjusted padding
- changed actions to buttons, using sysui button theming
- updated call to action text
- added speech bubble triangle

Also moves the onboarding so that it appears specifically above the first message in the shade, rather than above all notifications at the top.

Bug: 436251599
Test: manual
- revive onboarding & get a message notification for visual changes
- trigger a notification that appears above conversation section (e.g. screen recording) to confirm positioning
Flag: android.app.nm_summarization_onboarding_ui

Change-Id: I905e9a3add956dd2f91a103bf35641980851fe56