Add the Access token renewal functionality

[dubdabasoduba]

Context

• The MOMO API requires an Access Token when performing requests
• Write the functionality to authenticate and use that Access token when making requests to the API

Implementation

• Make sure the sample app requests the access tokens correctly from the server.
• Store the access token on the android account manager
• Write the functionality to renew the token whenever it expires.
• This should be triggered when the user displays the splash screen.

Acceptance Criteria

• The app should request an access token from the server
• The access token should be saved on the account manager
• The access token should be usable on the different requests that need it.