Root path in the preference manager won't update when workspace folder changes

[jdneo]

The updateWorkspaceFolders in https://github.com/eclipse/eclipse.jdt.ls/blob/master/org.eclipse.jdt.ls.core/src/org/eclipse/jdt/ls/core/internal/managers/ProjectsManager.java#L160

will not update the rootPaths in preferenceManager, which will cause some unexpected behavior when using rootPaths. for example, here:
https://github.com/eclipse/eclipse.jdt.ls/blob/master/org.eclipse.jdt.ls.core/src/org/eclipse/jdt/ls/core/internal/handlers/DocumentLifeCycleHandler.java#L309

Suppose we opened a multi-root workspace in VS Code. Then we added a new folder into the workspace, and the added folder contains an invisible project.

Because there is no trigger files information, the Invisible project importer won't work, and if we open its java source file, the Language server will open it as a default project file.

But if we correct the rootPaths field, the InvisibleProjectImporter will be triggered to import it.

[jdneo]

I can raise a PR to fix this.