71433 [flutter_tools] update some web configuration defaults (cla: yes, tool)

71434 [flutter_tools] allow hiding web server device, provide flags to re-enable (cla: yes, tool)

71439 [fluttter_tools] reland enable experimental invalidation strategy on by default in master channel (cla: yes, tool)

71446 Relands: Migrate template to Gradle 6.7 and AGP 4.1.0 (a: accessibility, cla: yes, d: examples, team, tool)

71451 Validate empty observatory URI for screenshot (cla: yes, tool, waiting for tree to go green)

71487 [flutter_tools] report basic analytics for null-safety (cla: yes, tool)

71491 [flutter_tools] hide web server by default (cla: yes, tool)