You can use scriblejava library to access Oauth 1.0 2-legged APIs.
In Android Studio App griddle add following dependency:
compile 'org.scribe:scribe:1.3.5'
And simply use following code:
String consumerKey = "XXXX"; //api key
String consumerSecret = "XXXX"; //api secret
String requestUrl = "your context.io request url";
OAuthService service = new ServiceBuilder()
.provider(OAuthProvider.class)
.apiKey(consumerKey)
.apiSecret(consumerSecret)
.build();
OAuthRequest request = new OAuthRequest(Verb.GET, requestUrl);
Token accessToken = new Token("", ""); //not required for context.io
service.signRequest(accessToken, request);
Response response = request.send();
Log.d("OAuthTask",response.getBody());
Hope it helped!