Default router serialization is not compatible with immutability check

[jonr-elsewhen]

Minimal reproduction of the bug/regression with instructions:

The "strictStateImmutability" runtime check is enabled by default in NgRx 8. This check is not compatible with @ngrx/router-store, because the default router state cannot be safely made immutable: #1104 (comment)

1. Create a Angular project without routing enabled.
2. Add @ngrx/store
3. Add @ngrx/router

Browse the app. You will get the following in the console:

Attempting to configure '__source' with descriptor '{"value":"StaticContentPipe","configurable":true}' on object 'InjectionToken STATIC_CONTENT_CONTEXT' and got error, giving up: TypeError: Cannot redefine property: __source

Expected behavior:

The "strictStateImmutability" check should skip freezing the router store by default.

Versions of NgRx, Angular, Node, affected browser(s) and operating system(s):

NgRx: 8
Angular: 8

I would be willing to submit a PR to fix this issue

[x] Yes
[ ] No

[timdeschryver]

To make router-store serializable you will have to use RouterState.Minimal.