[scenic] Convert story_shell APIs to ViewToken

We consume some SDK clients as prebuilts, so they must implement the new
API before we call into it.

Tested: No behavior changed; modular_tests, started story in sysUI
SCN-1018 #comment
SCN-1291 #comment

Change-Id: Id6a2354b942e5390fce855f81fd913f770423340