Fix text crossfade

When expanding a notification and changing the text displayed, fade-out
the going-away text earlier, and fade-in the new text later.

This avoids a state in between where some text is drawn on top of
other text.

Test: Create notification with long text, use drag to slowly expand
      notification. Check that text is never drawn on top of each other
Bug: 232852389
Change-Id: If0eb86aaf858ba7b2043bca56c4c70196709f078