Add ability to lazy render content for specific panel in case of nested expansion panels

[dbespalchuk]

Bug, feature request, or proposal:

Feature request

What is the expected behavior?

Only expansion panel that contains template with matExpansionPanelContent directive should lazy render it in case of a nested expansion panels.

What is the current behavior?

All parent expansion panels render matExpansionPanelContent template of child panel if they don't contain any specific template for themself.

What are the steps to reproduce?

StackBlitz reproduction: https://stackblitz.com/edit/angular-material2-issue-fa7h1c

What is the use-case or motivation for changing an existing behavior?

There is a case where I have to use nested expansion panels with lazy rendering for different content blocks in the child expansion panels. If I want to render content properly, then I also have to make all parent expansion panels content lazy rendered.

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

Angular 6.1.1
Material 6.4.2
TypeScript 2.9.2

Is there anything else we should know?

This issue can be also reproduced on the latest versions of Angular and Material

[VitalyVrublevskyy]

I have the same problem. Looks like cause of problem hidden here: https://github.com/angular/material2/blob/63be232283262615fa2130900c80dd570eff6cda/src/lib/expansion/expansion-panel.ts#L107:L108.
We should take into account only appropriate @ContentChild only for particular level of Accordion.

[kvloover]

ran into this as well, fix would be appreciated