Transcribe doesn't work in some regions with configuration Amplify CLI created

[tkashi]

Describe the bug When we configure Amplify app to use Amazon Transcribe in the Predictions category with a user in some regions, it doesn't work with the current AWS SDK actually. For example, as the below example, if we use ap-northeast-1 as a region, then Predictions.convert produces a WebSocket connection error.

Amplify CLI Version 4.17.2

To Reproduce

1. Using a user of ap-northeast-1, configure Transcribing text function via Amplify CLI with the default setting, which produces an aws-export.js like

   const awsmobile = {
   "aws_project_region": "ap-northeast-1",
   "aws_cognito_identity_pool_id": "ap-northeast-1:1111111111-b410-4aa8-be12-c234371bbb54",
   "aws_cognito_region": "ap-northeast-1",
   "aws_user_pools_id": "ap-northeast-1_xxxxxxxxx",
   "aws_user_pools_web_client_id": "xxxxxxxxxxxxxxxxxxxxxxx",
   "oauth": {},
   "predictions": {
       "convert": {
           "transcription": {
               "region": "ap-northeast-2",
               "proxy": false,
               "defaults": {
                   "language": "en-US"
               }
           }
       }
   }
   };

2. Create an app with amplify-sdk (@aws-amplify/predictions@3.1.6) which just runs Prediciton.convert({transcription}), e.g.

   
   import Amplify from '@aws-amplify/core';
   import Auth from '@aws-amplify/auth';
   import Predictions, {AmazonAIPredictionsProvider} from '@aws-amplify/predictions';
   import awsconfig from './aws-exports';

Amplify.configure(awsconfig); Amplify.addPluggable(new AmazonAIPredictionsProvider());

Auth.currentCredentials().then(() => { Predictions.convert({ transcription: { source: { bytes: [] } }, }); });

**Expected behavior**
The reason of this bug is because the current implementation of the AWS SDK is based on Streaming Transcription, but some regions (e.g. ap-northeast-1 and ap-northeast-2) don't support it. Amplify CLI should use another region if that of a user doesn't support Streaming Transcription.

**Screenshots**
<img width="568" alt="Screen Shot 2020-04-13 at 9 51 20" src="https://user-images.githubusercontent.com/1989039/79084566-68b8ba00-7d6f-11ea-9515-0aac62860d57.png">

**Desktop (please complete the following information):**
 - OS: Mac Mojave
 - Node: 12.16.1

**Additional context**
N/a

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