The height of the typing indicator could be the natural
height of the speech bubbles within the typing indicator.
However, the speech bubbles expand with an elastic curve.
This elasticity would be too visually jarring if it quickly 
pushed all the conversation messages up or down. Instead,
the height of the typing indicator animates on its own,
smoothly expanding before the bubbles appear.
When the bubbles disappear, the height smoothly contracts to zero. 
This behavior requires an explicit animation for the
height of the typing indicator.

Define an animation for the height of the typing indicator,
and then apply that animated value to the SizedBox
widget within the typing indicator.

The TypingIndicator runs an animation forward or backward
depending on whether the incoming showIndicator variable
is true or false, respectively.