vscode: Debug toolbar contributed action visibility not updated on context key change

Issue Type: Bug

Testing https://github.com/Microsoft/vscode/issues/69196

When setting a when clause on the command, it seems to be evaluated when the toolbar is shown, but then not revaluated when the underlying context key updates.

		"commands": [
			{
				"command": "extension.helloWorld",
				"title": "Hello World"
			}
		],
		"menus": {
			"debug/toolbar": [
				{
					"command": "extension.helloWorld",
					"group": "navigation",
					"when": "inZenMode"
				}
			]
		}

For this snippet, if starting debugging while in zen mode the command is shown, but toggling zen mode after that does not remove the command.

VS Code version: Code - Insiders 1.32.0-insider (393b48d4a5de85b6fa1d2ad0a59588d6715e12ea, 2019-02-25T01:33:51.804Z) OS version: Darwin x64 18.2.0