Below you can check out the video, but it appears as if Dwayne Johnson is stating his solo Black Adam film will come prior to the Shazam movie. Johnson says:
So what we decided to do was to create a scenario where Black Adam is having his own stand-alone movie, Captain Marvel, Shazam, will have his own stand-alone movie, and that way, we are building our world up that way, and then we can come together at some other point.
Dwayne Johnson also mentions a big Black Adam surpise is coming, which could be an announcement of the Black Adam movie (update: It could be Dwayne Johnson in Man of Steel 2).
My two cents: I think it's a smart move to go with the Dwayne Johnson Black Adam movie first. Right now Johnson is arguably the top male actor in Hollywood, so to have him headline and star in his own DCEU movie makes sense. In addition, with a solo Black Adam movie, they can really develop the character instead of throwing him into what essentially would be "Batman vs. Superman Part 2," but on a bigger scale with a Shazam vs Black Adam flick. It also sounds like they've placed the Shazam movie possibly on the backburner a bit, which may mean they won't have to cast the actor any time soon.
Here's the text of what Johnson had to say:
"The cool think about that is we've had great discussions with Geoff Johns over at DC. Phenomenal. This is a really fun, cool time for DC right now because they're really in a process of truly world-building, and building out. We're seeing that with Wonder Woman and Aquaman. We have a few surprises down the line."
"So what we decided to do was to create a scenario where Black Adam is having his own stand-alone movie, Captain Marvel, Shazam, will have his own stand-alone movie, and that way, we are building our world up that way, and then we can come together at some other point."
"We have a surprice for Black Adam coming up. I can't tell you what it is. It is going to be... The surprise for Black Adam is going to be... It's just fucking awesome."