Use default random provider on Windows

bramble-core/src/main/java/org/briarproject/bramble/system/WindowsSecureRandomProvider.java

+package org.briarproject.bramble.system;
+
+import javax.annotation.Nullable;
+import java.security.Provider;
+
+public class WindowsSecureRandomProvider extends AbstractSecureRandomProvider {
+    @Nullable
+    @Override
+    public Provider getProvider() {
+        return null;
+    }
+}

bramble-java/src/main/java/org/briarproject/bramble/system/DesktopSecureRandomModule.java

@@ -9,6 +9,7 @@ import dagger.Provides;
 
 import static org.briarproject.bramble.util.OsUtils.isLinux;
 import static org.briarproject.bramble.util.OsUtils.isMac;
+import static org.briarproject.bramble.util.OsUtils.isWindows;
 
 @Module
 public class DesktopSecureRandomModule {
@@ -18,7 +19,8 @@ public class DesktopSecureRandomModule {
 	SecureRandomProvider provideSecureRandomProvider() {
 		if (isLinux() || isMac())
 			return new UnixSecureRandomProvider();
-		// TODO: Create a secure random provider for Windows
+		if (isWindows())
+			return new WindowsSecureRandomProvider();
 		throw new UnsupportedOperationException();
 	}
 }