saveAccessToken
Persists the Bearer access token along with its expiry time and token type.
The expiry is calculated from AccessToken.expiresIn (seconds) and stored as an absolute epoch millisecond value so that getAccessToken can check it without needing to know when the token was first saved.
Parameters
token
The AccessToken received from the MTN MoMo token endpoint, or null (in which case this call is a no-op).