[tf-signal] Add inverse_stft_window_fn to tf.contrib.signal.

To reconstruct an original waveform, a complimentary window function should
be used in inverse_stft. Such a window function can be constructed with
tf.contrib.signal.inverse_stft_window_fn.

Inlines and refactors the test for equivalence of inverse_stft and original
waveform.  Adds a test case that shows a large difference between inverse stft
with and without complimentary window function.

PiperOrigin-RevId: 174530978