Each bubble appears by animating its scale from 0% to 100%,
and each bubble does this at slightly different times so
that it looks like each bubble appears after the one before it.
This is called a staggered animation.

Paint the three bubbles in the desired positions from the
lower left. Then, animate the scale of the bubbles
so that the bubbles are staggered whenever the showIndicator
property changes.

Animate the flashing circles

Within the large speech bubble, the typing indicator
displays three small circles that flash repeatedly.
Each circle flashes at a slightly different time,
giving the impression that a single light source is 
moving behind each circle. This flashing animation
repeats indefinitely.

Introduce a repeating AnimationController to
implement the circle flashing and pass it to the
StatusBubble.