Use null-safety annotations on buildSrc code

buildSrc/build.gradle

+repositories {
+    mavenCentral()
+}
+
+dependencies {
+    api 'org.briarproject:null-safety:0.1'
+}
\ No newline at end of file

buildSrc/src/main/java/org/briarproject/onionwrapper/OS.java

 package org.briarproject.onionwrapper;
 
+import org.briarproject.nullsafety.NotNullByDefault;
+
+@NotNullByDefault
 public enum OS {
 	Linux("linux"),
 	Windows("windows"),

buildSrc/src/main/java/org/briarproject/onionwrapper/OsUtils.java

 package org.briarproject.onionwrapper;
 
+import org.briarproject.nullsafety.NotNullByDefault;
+
 import static org.briarproject.onionwrapper.StringUtils.startsWithIgnoreCase;
 
+@NotNullByDefault
 public class OsUtils {
 
 	public static final OS currentOS;

buildSrc/src/main/java/org/briarproject/onionwrapper/StringUtils.java

 package org.briarproject.onionwrapper;
 
+import org.briarproject.nullsafety.NotNullByDefault;
+
+@NotNullByDefault
 public class StringUtils {
 
 	// see https://stackoverflow.com/a/38947571