Support 2-way sync between desktop and mobile

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**Describe the bug**
Sync function remove my other accounts

**To Reproduce (REQUIRED)**
1. Create the first account on the desktop, using MetaMask extension
2. Import the account above on the mobile, using MetaMask Andriod app
3. Create 3 more accounts using the MetaMask Andriod app, in the meantime, the desktop remains 1 account, the original account created on step1
4. Execute the Sync function on the desktop, using MetaMask Extension
5. The 3 newly created accounts got erased, only the original account remains

**Expected behavior**
Sync function should sync 2 ways: a. desktop -> mobile b. mobile -> desktop
Sync function should always on: when I create a new account on the mobile, the desktop should also be updated automatically

**Browser details (please complete the following information):**
- OS: Windows 10, Andriod 9
- Browser : Brave, Version 1.2.43 Chromium: 79.0.3945.130 (Official Build) (64-bit)
- MetaMask Extension Version: 7.7.3

Start by tracing the desktop Sync function described in the reproduction steps, then compare how accounts created on desktop and Android mobile are represented and transferred. Done means syncing preserves accounts created on either device, supports both directions, and updates the other device when a new account is created.

Written by the indexing model from the issue text.