[Bug] Users % targettig does not seem to work as one could think is does

### [REQUIRED] Please fill in the following fields:

* Pre-built SDK from the [website](https://firebase.google.com/download/cpp) or open-source from this repo: Open source from this repo
* Firebase C++ SDK version: 8.9.0
* Problematic Firebase Component: Remote config
* Other Firebase Components in use: None
* Platform you are using the C++ SDK on: Windows, Mac
* Platform you are targeting: desktop

### [REQUIRED] Please describe the issue here:

Users % targettig does not seem to work as it should. It rolls a dice every fetch so it does not have any notion of a user or client thus single client can get various configs every fetch

#### Steps to reproduce:

Configure remote config so it has a user % based condition and serve one config (config A) for 50% of users and the other (config B) for the rest. Fetch the config multiple times.
Expected: A user who got config A keeps getting config A and the one who got config B always gets config B.
Actual: The user who got config A gets config B on subsequent fetch.

#### Relevant Code:

No files or tests are named. Start by locating the C++ Remote Config implementation for user-percentage conditions and reproduce repeated fetches with a 50/50 configuration on Windows or Mac. Done means a client that receives config A or B continues receiving the same assignment across subsequent fetches.

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