Until now the users were forced to subclass QAndroidService in order to
provide the binder. Now is much easier, the user just pass a lambda in
the QAndroidService constructor e.g.

QAndroidService app(argc, argv, [](const QAndroidIntent &){ return new
MyBinder{};});

Change-Id: I97608f806b311ad3c853a86cde132aea8352349b
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>