BidirectionalSequenceLSTM op: Adds layer norm support.

Also updates documentation for this op and does some minor clean-up for
UnidirectionalSequenceLSTM op (documentation and reducing the number of
unnecessary decimal places for test data).

Bug: 123644584

Test: NeuralNetworksTest_static --gtest_filter=GeneratedTests.*lstm*
Test: NeuralNetworksTest_static --gtest_filter=OperationValidationTest.*LSTM*

Change-Id: I62a6ac49ebdbcebe9eb986568cd2d17686bd5809