Exception java.lang.NoSuchMethodError: decodeBase64 in Android devices with API < 28

Hi,
We have found that the library is throwing an exception in any Android device with and API < 28 (Android 9.0) after version `com.google.cloud:google-cloud-speech:0.70.0-beta`. The problem is related with [this issue](https://issuetracker.google.com/issues/37033030). As far as we know, [Apache common codec library](http://commons.apache.org/proper/commons-codec/) dependency version is 1.10 in this project, but Android in versions below 9.0 has the same classes in boot classpath, so classes from the version 1.10 will never be loaded. This issue will not be solved in Android, thus we would like to ask for a solution here, removing/replacing the dependency in some way.
Thanks in advance.

Please include as much information as possible:

#### Environment details

- OS: Android API < 28
- Java version:1.8
- google-cloud-java version(s): from 0.71.0-beta

#### Steps to reproduce

1\. Zip file attached or get Android speech example source code from:
https://github.com/GoogleCloudPlatform/android-docs-samples/tree/master/speech/SpeechRecognitionClient

2\. Import app into Android Studio.

3\. Modify build-gradle (module app) file to update dependencies in
section dependencies:
```
// add these dependencies for the speech client
implementation 'io.grpc:grpc-okhttp:1.18.0'
implementation 'com.google.cloud:google-cloud-speech:0.71.0-beta'
```
4\. Build app with Android Studio and run in development mode on Android device below API 28.

#### Stacktrace

```
2019-02-21 10:56:28.171 12310-12310/? E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.google.cloud.examples.speechrecognition, PID: 12310
java.lang.NoSuchMethodError: No static method decodeBase64(Ljava/lang/String;)[B in class Lorg/apache/commons/codec/binary/Base64; or its super classes (declaration of 'org.apache.commons.codec.binary.Base64' appears in /system/framework/org.apache.http.legacy.boot.jar)
at com.google.api.client.util.Base64.decodeBase64(Base64.java:101)
at com.google.api.client.util.PemReader.readNextSection(PemReader.java:106)
at com.google.api.client.util.PemReader.readFirstSectionAndClose(PemReader.java:135)
at com.google.auth.oauth2.ServiceAccountCredentials.privateKeyFromPkcs8(ServiceAccountCredentials.java:290)
at com.google.auth.oauth2.ServiceAccountCredentials.fromPkcs8(ServiceAccountCredentials.java:280)
at com.google.auth.oauth2.ServiceAccountCredentials.fromJson(ServiceAccountCredentials.java:210)
at com.google.auth.oauth2.GoogleCredentials.fromStream(GoogleCredentials.java:174)
at com.google.auth.oauth2.GoogleCredentials.fromStream(GoogleCredentials.java:141)
at com.google.cloud.examples.speechrecognition.MainActivity$mSpeechClient$2$1$1.getCredentials(MainActivity.kt:69)
at com.google.cloud.examples.speechrecognition.MainActivity$mSpeechClient$2$1$1.getCredentials(MainActivity.kt:47)
at com.google.api.gax.rpc.ClientContext.create(ClientContext.java:132)
at com.google.cloud.speech.v1.stub.GrpcSpeechStub.create(GrpcSpeechStub.java:94)
at com.google.cloud.speech.v1.stub.SpeechStubSettings.createStub(SpeechStubSettings.java:126)
at com.google.cloud.speech.v1.SpeechClient.(SpeechClient.java:144)
at com.google.cloud.speech.v1.SpeechClient.create(SpeechClient.java:126)
at com.google.cloud.examples.speechrecognition.MainActivity$mSpeechClient$2.invoke(MainActivity.kt:68)
at com.google.cloud.examples.speechrecognition.MainActivity$mSpeechClient$2.invoke(MainActivity.kt:47)
at kotlin.SynchronizedLazyImpl.getValue(LazyJVM.kt:74)
at com.google.cloud.examples.speechrecognition.MainActivity.getMSpeechClient(MainActivity.kt)
at com.google.cloud.examples.speechrecognition.MainActivity.onResume(MainActivity.kt:105)
at android.app.Instrumentation.callActivityOnResume(Instrumentation.java:1270)
at android.app.Activity.performResume(Activity.java:6830)
at android.app.ActivityThread.performResumeActivity(ActivityThread.java:3407)
at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:3470)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1528)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6121)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:889)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:779)

```
#### Code snippet

```
private val mSpeechClient by lazy {
// NOTE: The line below uses an embedded credential (res/raw/sa.json).
// You should not package a credential with real application.
// Instead, you should get a credential securely from a server.
applicationContext.resources.openRawResource(R.raw.google_cloud_credentials).use {
SpeechClient.create(SpeechSettings.newBuilder()
.setCredentialsProvider { GoogleCredentials.fromStream(it) }
.build())
}
}
```

#### Any additional information below

Thanks!