SDL encrypts payload of 1st frame of multi-frame message

[dboltovskyi]

Bug Report

In case if RPC service has switched to protected mode and SDL sends multi-frame message payload of 1st frame is encrypted. However it shouldn't be encrypted and it's size should always be 8 bytes. (See #2384)

Reproduction Steps

1. Start SDL, HMI, connect mobile device
2. Register new App
3. Switch RPC service to protected mode
   => SDL starts PTU through protected channel and sends encrypted OnSystemRequest notification to mobile with Policy Table Snapshot (PTS). In case if PTS is >16kb it would be split into a few frames.
4. Check content of the 1st frame

Expected Behavior

• Payload is non-encrypted
• Size is 8 bytes

Observed Behavior

• Payload is encrypted
• Size is greater than 8 bytes (30-50 bytes)

OS & Version Information

• OS/Version: Ubuntu 20.04
• SDL Core Version: https://github.com/smartdevicelink/sdl_core/tree/develop (fed2ae4)
• Testing Against: ATF

Test Case, Sample Code, and / or Example App

• ATF Script: 3640_PTU_all_flows_Protected_Mode_v3
• Report and Logs: 2021-03-11_09-22-49.555_3659.tar.gz
• Snippet from extended console log:

Decode:	2	true	45	8	20522	2
[14:23:00,243] SDL->MOB:  N:	OnSystemRequest (encrypted)

`2` - first frame
`true` - encryption flag
`45` - size of the payload

[LitvinenkoIra]

Luxoft team is going to provide a fix.