[securitycenter] v2 client's `apiEndpoint` property is wrong when a regional endpoint is specified

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### A screenshot that you have tested with "Try this API".

n/a

### Link to the code that reproduces this issue. A link to a **public** Github Repository or gist with a minimal reproduction.

https://gist.github.com/hegemonic/cf807026d8b83c14aa0b225b16967599

### A step-by-step description of how to reproduce the issue, based on the linked reproduction.

Run the code. It creates a v2 `SecurityCenterClient`; sets the `apiEndpoint` to `securitycenter.me-central2.rep.googleapis.com`; and prints the expected and actual `apiEndpoint` for the client.

### A clear and concise description of what the bug is, and what you expected to happen.

The code prints the following:

```
expected apiEndpoint: securitycenter.me-central2.rep.googleapis.com
actual apiEndpoint: securitycenter.googleapis.com
client._opts.apiEndpoint: securitycenter.me-central2.rep.googleapis.com
```

I expected it to print the following:

```
expected apiEndpoint: securitycenter.me-central2.rep.googleapis.com
actual apiEndpoint: securitycenter.me-central2.rep.googleapis.com
client._opts.apiEndpoint: securitycenter.me-central2.rep.googleapis.com
```

### A clear and concise description WHY you expect this behavior, i.e., was it a recent change, there is documentation that points to this behavior, etc. **

* If I specify an API endpoint, the client object shouldn't say that it's using the default endpoint.
* You shouldn't need to look at private properties to figure out which endpoint the client is using.