Update jdk and old workspace still points to the stale JRE system libraries

Steps to reproduce:

1. Install jdk1.8.0_131 in windows 10, and set JAVA_HOME and Path environment variables.

2. Install latest vscode-java extension in vscode, and open a java 8 maven project.

3. F12 at java.io.PrintStream.println, jump to editor contents/rt.jar/java.io/PrintStream.class.

4. Install the latest jdk-9.0.1 and update JAVA_HOME and Path environment variables.

5. Restart vscode window.

6. F12 at java.io.PrintStream.println, still jump to the stale jre source contents/rt.jar/java.io/PrintStream.class. It's supposed to jump to the java 9 source contents/java.base/java.io/PrintStream.class.

Actually the java language server was launched by the new java 9 jvm, but the workspace cached the old JRE 8 system libraries and user has no way to change the JRE system libraries.

Current workaround is to clean up the vscode local storage directory C:\Users\jinbwan\AppData\Roaming\Code - Insiders\User\workspaceStorage.

Expected:
When vscode-java extension startup, update the workspace jre container to the one under JAVA_HOME silently.
Or provide a command to allow user to update the workspace JRE container.