[desktop] performing a hot reload should focus on the app

## Observed result
1) Launch a macos flutter app from the terminal
1) Change the code
1) go to the terminal and perform a hot reload
1) Notice now that you have to find the app and bring it to focus to see the results with something like alt+tab.

## Expected result
I'd expect that performing a hot reload would bring the app into focus when performing a hot reload.

## Proposal
Focusing on the app should be the default behavior for the flutter tool when developing desktop apps. We can add a flag `--no-auto-focus` to override that behavior in case someone ever doesn't like it.