Does the app on that phone have a "manual" or "advanced" mode? If it has the option will be in there, if it exists. Since that phone has a fixed aperture if you can also fix the ISO then being able to set the shutter speed manually is equivalent to fixing the exposure. If there isn't a manual setting, look for an "exposure lock" which would give you another way of doing this (move the camera around or apply exposure compensation until the exposure is what you want, lock the exposure, recompose and take picture(s)).
Open Camera has an exposure lock option (a little padlock symbol), though not a manual exposure setting.
Some camera apps also adjust the exposure depending on where the focus spot is, so tapping a particular point on the screen may change the exposure. Depending on your needs for the particular shot that may or may not help.
(It's not the "android" camera app you have, it's the Samsung app for that camera. Most manufacturers have their own camera apps, and even Google phones now have a special Google app rather than a generic Android one. Hence I'm not sure whether there really is an android camera app any more - if there is it will be the old AOSP one, probably not developed for years).
