The problem comes from saving a project in Projucer on Windows, because it writes some garbage string in Builds/Android/app/CMakeLists.txt. The DemoRunner example from JUCE was saved on macOS or Linux, so there is no garbage in its Builds/Android/app/CMakeLists.txt file. If you open the DemoRunner example in Projucer on Windows and re-save it, you should have the same problem.
See this other thread for some details about why a garbage string is written on Windows: String assertion in android build.