Switch to GCM CCS and add support for APN feedback processing.

src/main/java/org/whispersystems/textsecuregcm/configuration/GcmConfiguration.java

@@ -19,8 +19,14 @@ package org.whispersystems.textsecuregcm.configuration;
 import com.fasterxml.jackson.annotation.JsonProperty;
 import org.hibernate.validator.constraints.NotEmpty;
 
+import javax.validation.constraints.NotNull;
+
 public class GcmConfiguration {
 
+  @NotNull
+  @JsonProperty
+  private long senderId;
+
   @NotEmpty
   @JsonProperty
   private String apiKey;
@@ -28,4 +34,8 @@ public class GcmConfiguration {
   public String getApiKey() {
     return apiKey;
   }
+
+  public long getSenderId() {
+    return senderId;
+  }
 }