java.lang.NoClassDefFoundError: java.nio.charset.StandardCharsets in Android API < 19

Hi,

we have problems with the latest version of your library in devices running Android 4.3 (API 18) or lower. `StandardCharsets` was added in Android API 19 so any call to methods using that (for example, `StringUtils.getBytesUtf8` or `StringUtils.newStringUtf8`) causes a crash in our app.

#### Environment details

- OS: **Android API < 19**
- google-http-java-client version: **1.28**

#### Stacktrace

```
java.lang.NoClassDefFoundError: java.nio.charset.StandardCharsets
at com.google.api.client.util.StringUtils.getBytesUtf8(StringUtils.java:52)
at com.google.api.client.json.webtoken.JsonWebSignature.signUsingRsaSha256(JsonWebSignature.java:636)
at com.google.api.client.googleapis.auth.oauth2.GoogleCredential.executeRefreshToken(GoogleCredential.java:388)
at com.google.api.client.auth.oauth2.Credential.refreshToken(Credential.java:494)
at com.google.api.client.auth.oauth2.Credential.intercept(Credential.java:217)
at com.google.api.client.googleapis.batch.BatchRequest$BatchInterceptor.intercept(BatchRequest.java:281)
at com.google.api.client.http.HttpRequest.execute(HttpRequest.java:911)
at com.google.api.client.googleapis.batch.BatchRequest.execute(BatchRequest.java:233)
at our.app.PubSubManager.sendTopic(PubSubManager.java:100)
at java.lang.Thread.run(Thread.java:841)
```

#### Any additional information below

https://developer.android.com/reference/java/nio/charset/StandardCharsets.html

####

Thanks!