Correctly compute the center of the time picker (#6323)
Correctly compute the center of the time picker (#6323)

Previously we assumed the dial was square, which isn't always the case.

Fixes #6315
tvolkert/flutter,Hixie/flutter,tvolkert/flutter,Hixie/flutter,Hixie/flutter,turnidge/flutter,flutter/flutter,turnidge/flutter,cbracken/flutter,cbracken/flutter,flutter/flutter,Hixie/flutter,turnidge/flutter,cbracken/flutter,Hixie/flutter,cbracken/flutter,flutter/flutter,tvolkert/flutter,flutter/flutter,flutter/flutter,cbracken/flutter,flutter/flutter,cbracken/flutter,flutter/flutter,cbracken/flutter,tvolkert/flutter,tvolkert/flutter,tvolkert/flutter,tvolkert/flutter,Hixie/flutter,tvolkert/flutter,Hixie/flutter,cbracken/flutter,flutter/flutter,Hixie/flutter,tvolkert/flutter,Hixie/flutter,flutter/flutter,turnidge/flutter,flutter/flutter,Hixie/flutter,cbracken/flutter,cbracken/flutter,tvolkert/flutter,turnidge/flutter