[Add area] Show polygon area for longer

Upon completion of drawing a polygon, the size of the polygon is briefly shown at the bottom of the screen for a second in squaremeters m².

**Feature Request**
Users like the feature, but lament that it disappears too quickly.
Possible solutions:
- Make the pop-up stay longer.
- Instead of a pop-up, show the size next to the polygon (where in above screenshot you see the length of the last edge "169ft").
- Show the size on the last screen (see example).

This FR partially relates to https://github.com/google/ground-android/issues/3351.