This reverts commit b63aff33.

Reason for revert: Breaking out-of-tree session shell

Original change's description:
> [scenic] Convert session_shell to ViewToken
> 
> Tested: No behavior changed; modular_tests, started story in sysUI
> SCN-1018 #comment
> SCN-1291 #comment
> 
> Change-Id: I8509501a15094d7d381557c186a9d0463488a956

TBR=mesch@google.com,thatguy@google.com,lindkvist@google.com,dworsham@google.com,pascallouis@google.com

Change-Id: If1fb32ae78f28c66eca4124fa0bcb52f62422fb7
No-Presubmit: true
No-Tree-Checks: true
No-Try: true