fix(@schematics/angular): fix app shell schematic failure

[devoto13]

It fails fails with Chunk cannot be found. error message, when AppModule already has import for RouterModule. The reason is that schematic code was assuming that it only receives InsertChange, but addImportToModule also may return NoopChange, when import is already present.

Fixes #10093

[alan-agius4]

Thanks, LGTM

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