Migrate RecipientDatabase to Kotlin.

app/src/main/java/org/thoughtcrime/securesms/util/Base64.java

@@ -1,6 +1,7 @@
 package org.thoughtcrime.securesms.util;
 
 import androidx.annotation.NonNull;
+import androidx.annotation.Nullable;
 
 import java.io.IOException;
 
@@ -24,4 +25,16 @@ public final class Base64 {
       throw new AssertionError();
     }
   }
+
+  public static @Nullable byte[] decodeNullableOrThrow(@Nullable String s) {
+    if (s == null) {
+      return null;
+    }
+
+    try {
+      return org.whispersystems.util.Base64.decode(s);
+    } catch (IOException e) {
+      throw new AssertionError();
+    }
+  }
 }