update-report: further improve analytics messaging

- Don't output the analytics message and mark it as read if
`HOMEBREW_NO_ANALYTICS_THIS_RUN` is set. This mostly simplifies the
installer code where we can display the message ourselves there rather
than having `brew update` print it out sometimes and not others (i.e.
when there's no system Git installed).
- Use the shell's audible bell to nudge people to actually read this
message (and give them less excuse to complain when they don't).
- Add an extra newline and bold all the analytics messaging so it's
more visible in the output.