[java-firestore] CustomClassMapper does not handle well getters that are java bridge methods

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#### Environment details

1. Specify the API at the beginning of the title. For example, "BigQuery: ...").
General, Core, and Other are also allowed as types
2. OS type and version:
3. Java version:
4. firestore version(s):

#### Steps to reproduce

Mirroring from https://github.com/firebase/firebase-admin-java/issues/435
1. ?
2. ?

#### Code example

IMHO it worth adding following code to com.google.cloud.firestore.CustomClassMapper.BeanMapper#shouldIncludeGetter:
// Bridge methods
if (method.isBridge()) {
return false;
}

#### Stack trace
```
Any relevant stacktrace here.
```

#### External references such as API reference guides

- ?

#### Any additional information below

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