Update the extension activate event

[jdneo]

Current the extension will be activated only if the extension command is triggered. Or user opened a java file.

We need to consider to change the activation event. For example https://github.com/redhat-developer/vscode-java/blob/master/package.json

Here are some changes that need to be done:

• Activate the extension when the workspace contains pom.xml/build.gradle
• Activate the extension when test explorer gets focus