util: Extract non-Android utility interfaces

As part of a refactoring that will likely introduce more custom
collection classes, move the non-Android-specific parts outside the
com.wireguard.android package.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

app/src/main/java/com/wireguard/android/model/Tunnel.java

@@ -13,7 +13,7 @@ import android.support.annotation.Nullable;
 
 import com.wireguard.android.BR;
 import com.wireguard.android.util.ExceptionLoggers;
-import com.wireguard.android.util.Keyed;
+import com.wireguard.util.Keyed;
 import com.wireguard.config.Config;
 
 import java.util.regex.Pattern;