Clean up properties of layers.Layer:

* Make `activity_regularizer` a real read-only property settable by
  the constructor.
* Make `name` a read-only property instead of mutable.
* Make `inbound_nodes`, `outbound_nodes`, `batch_input_shape` private.

Also: Update the documentation of Layer to indicate that it is stable,
and include guidance for how to use it.
PiperOrigin-RevId: 170777368