[Compose Notifs] Center title & expander when we have no content

When the notification has only one line (sometimes referred to as
"header-only" in the legacy codebase), we want that line and the
expander to be centered vertically.

Bug: 431222735
Test: manual in the Gallery app
Flag: EXEMPT not production code yet
Change-Id: Ib22d78747586250517d7cb4c0e4101d4bd3312d2