[android] - add Keep annotations to abstract components

platform/android/MapboxGLAndroidSDK/src/main/java/com/mapbox/mapboxsdk/http/NativeHttpRequest.java

@@ -5,6 +5,7 @@ import com.mapbox.mapboxsdk.Mapbox;
 
 import java.util.concurrent.locks.ReentrantLock;
 
+@Keep
 public class NativeHttpRequest implements HttpResponder {
 
   private final HttpRequest httpRequest = Mapbox.getModuleProvider().createHttpRequest();

platform/android/MapboxGLAndroidSDK/src/main/java/com/mapbox/mapboxsdk/log/Logger.java

@@ -1,5 +1,6 @@
 package com.mapbox.mapboxsdk.log;
 
+import android.support.annotation.Keep;
 import android.util.Log;
 
 /**
@@ -9,6 +10,7 @@ import android.util.Log;
  * Alternative implementations can be set with {@link #setLoggerDefinition(LoggerDefinition)}.
  * </p>
  */
+@Keep
 public final class Logger {
 
   private static final LoggerDefinition DEFAULT = new LoggerDefinition() {