Add null-safety and thread-safety annotations.

bramble-android/src/main/java/org/briarproject/bramble/network/AndroidNetworkManager.java

@@ -13,6 +13,8 @@ import org.briarproject.bramble.api.lifecycle.Service;
 import org.briarproject.bramble.api.network.NetworkManager;
 import org.briarproject.bramble.api.network.NetworkStatus;
 import org.briarproject.bramble.api.network.event.NetworkStatusEvent;
+import org.briarproject.bramble.api.nullsafety.MethodsNotNullByDefault;
+import org.briarproject.bramble.api.nullsafety.ParametersNotNullByDefault;
 import org.briarproject.bramble.api.system.Scheduler;
 
 import java.util.concurrent.Future;
@@ -22,6 +24,7 @@ import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicReference;
 import java.util.logging.Logger;
 
+import javax.annotation.Nullable;
 import javax.inject.Inject;
 
 import static android.content.Context.CONNECTIVITY_SERVICE;
@@ -35,6 +38,8 @@ import static java.util.concurrent.TimeUnit.MINUTES;
 import static java.util.concurrent.TimeUnit.SECONDS;
 import static java.util.logging.Level.INFO;
 
+@MethodsNotNullByDefault
+@ParametersNotNullByDefault
 class AndroidNetworkManager implements NetworkManager, Service {
 
 	private static final Logger LOG =
@@ -122,7 +127,7 @@ class AndroidNetworkManager implements NetworkManager, Service {
 			}
 		}
 
-		private boolean isSleepOrDozeEvent(String action) {
+		private boolean isSleepOrDozeEvent(@Nullable String action) {
 			boolean isSleep = ACTION_SCREEN_ON.equals(action) ||
 					ACTION_SCREEN_OFF.equals(action);
 			boolean isDoze = SDK_INT >= 23 &&
@@ -130,7 +135,7 @@ class AndroidNetworkManager implements NetworkManager, Service {
 			return isSleep || isDoze;
 		}
 
-		private boolean isApEvent(String action) {
+		private boolean isApEvent(@Nullable String action) {
 			return WIFI_AP_STATE_CHANGED_ACTION.equals(action);
 		}
 	}

bramble-api/src/main/java/org/briarproject/bramble/api/network/NetworkManager.java

 package org.briarproject.bramble.api.network;
 
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+@NotNullByDefault
 public interface NetworkManager {
 
 	NetworkStatus getNetworkStatus();

bramble-api/src/main/java/org/briarproject/bramble/api/network/NetworkStatus.java

 package org.briarproject.bramble.api.network;
 
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.Immutable;
+
+@Immutable
+@NotNullByDefault
 public class NetworkStatus {
 
 	private final boolean connected, wifi;

bramble-api/src/main/java/org/briarproject/bramble/api/network/event/NetworkStatusEvent.java

@@ -2,7 +2,12 @@ package org.briarproject.bramble.api.network.event;
 
 import org.briarproject.bramble.api.event.Event;
 import org.briarproject.bramble.api.network.NetworkStatus;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
+import javax.annotation.concurrent.Immutable;
+
+@Immutable
+@NotNullByDefault
 public class NetworkStatusEvent extends Event {
 
 	private final NetworkStatus status;