[Notif redesign] Update fonts of text in the shade

This change updates the fonts for:
- the "Clear all" button and "Unlock to see older notifications" text
- the "No notifications" text
- the "Silent" section header

Note that the new "TextAppearance.NotificationShadeText" style is
identical to "TextAppearance.DeviceDefault.Widget.Button" when the flag
is off, which makes it equivalent with textAppearanceButton. This
ensures that there are no font changes when the flag is off.

Bug: 409496809
Test: manual
Flag: android.app.notifications_redesign_fonts
Change-Id: Id729421f47a9acd0fce4fda87f215b081f6a5fdb