JFR: Turn off events for JFR.view

Reproducer:

Shell 1:
$ java -XX:StartFlightRecording:exceptions=all -jar J2Demo.jar

Shell 2:
$ jcmd J2Demo.jar JFR.view JavaExceptionThrow

Result is exceptions from the view command, for example ParseException. We should disable events for the Attach thread while DCmdView executes. The API for doing this already exists.