mesh: Remove redundant code from mesh/crypto

mesh: Remove redundant code from mesh/crypto

- Remove application_encrypt/application_decrypt
- Make mesh_crypto_privacy_counter private, add mesh_crypto_pecb
- Make *_nonce functions private and align their implementation to be
  more consistent
- Refactor network_encrypt/network_decrypt to use *_nonce functions and
  rename them to packet_encrypt/packet_decrypt
- Refactor packet_encode/packet_decode

Change-Id: If342e994774714a75bdd0ae90766d1ec7b750b79
Signed-off-by: Abhay Agarwal <ay.agarwal@samsung.com>