Makes sure Keras Layer's `__call__` is always used in Eager.

Currently if a Layer is invoked with the Functional API in Eager, `__call__` is only used
during setup, and thereafter `call` is used internally. This limits the ability
to add pre/post processing steps to `call` in Eager in the future.
Additionally, the Subclassed Model API already always uses `__call__` in Eager.

PiperOrigin-RevId: 215778408