Use explicit String constants for OnlineState

[schmidt-sebastian]

@Feiyang1 noticed in a sample app that multitab is not able to parse OnlineStates since the constants are rewritten to __PRIVATE_OnlineState:

@firebase/firestore: Firestore (7.9.1): SharedClientState Failed to parse online state: {"clientId":"Xnj51M6igYRWov1EHcwt","onlineState":"__PRIVATE_Unknown"}

This PR removes the enum usage for OnlineState, to make sure that the constant names are not mangled.

Multi-Tab uses the same concept for its other "enums":

firebase-js-sdk/packages/firestore/src/local/shared_client_state.ts

Line 209 in bb1f4de

['pending', 'acknowledged', 'rejected'].indexOf(mutationBatch.state) !==

As a guide, the size impact of this PR:

• +7 bytes versus master (before GZIP)
• -43 bytes versus master (after GZIP) WHEEE

[wilhuff]

LGTM

This is fine as an intermediate state while we work out exactly how to address enums.