ApplicationService.currentAppId$ does not emit for legacy application

[pgayvallet]

The currentAppId$ is currently updated in the NP apps mount function

kibana/src/core/public/application/application_service.tsx

Lines 171 to 175 in de7a22d

	const mount: AppMounter = async params => {
	const unmount = await handler(params);
	this.currentAppId$.next(app.id);
	return unmount;
	};

• It will never trigger for legacy apps that uses a page refresh to display
• It seems like even for NP app, mount it's not called on initial display (need to confirm)

currentAppId$ should emit for the initially displayed app as soon as core got the information, either from the service itself or from the AppRouter.

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)