[tf.data] Turn the `map_parallelization` optimization on by default.

This CL:
- turns the `map_parallelization` optimization on by default
- fixes a bug in the optimization code that would result in a segfault if two consecutive map transformations were both parallelized
- fixes a bug in the OptimizationOptions._static_optimizations() method that would cause `apply_default_optimization = False` to override explicitly enabled default optimizations

PiperOrigin-RevId: 229237263