Maybe you have already used Lcamera with longer exposure time
It allows to make great night photos, but this camera has a fairly limited functionality.
So, i took the source code of fully featured Open Camera and did the same - took away exposure time restrictions.
Keep in mind:
- application is unstable with long exposure time
- preview has max exposure time 1/12 sec
- tested only on Nexus 6P, Android 8.0.0
- I changed only a few lines of code and forced to use camera2 API by default
Stock Normal - Stock HDR+ - Open Camera (5 sec, ISO400)
EDIT: Enabling manual exposure control in Open Camera: