Modify Sync UI to allow only one device in chain

[cezaraugusto]

Close brave/brave-browser#6941

This PR also reverts a3708ba cc @AlexeyBarabash

Test Plan

• Users should be able to start a Sync chain with one device
• After enabling Sync with only one device, a popup named "choose device type" should be rendered and persist at every page refresh.

[cezaraugusto]

@darkdh @AlexeyBarabash this PR is ready for review

[AlexeyBarabash]

Previous message is wrong because I tested on the wrong branch.

[AlexeyBarabash]

After proper update of the branch, two things for the case when 1 device is in chain remains.
STR:

1. Create Sync chain on deviceA, choose Computer - OK
2. Copy code from deviceA and connect deviceB to the chain with the code
   Actual: on deviceB I see dialog [Phone/Tablet Computer] if I choose Computer and click Cancel - I can see two devices
   Expected: on deviceB the dialog [Phone/Tablet Computer] should be closed once there are two devices in list.
3. Click Leave Sync Chain on deviceB and confirm
4. Wait until deviceA gets the changes
   Actual: deviceA displays list of 1 device
   Expected: deviceA should show Add new device dialog
5. Refresh brave://sync/ page on deviceA - OK, seeing Add new device dialog
6. On deviceA press Add New Device => Computer and click Cancel - OK, chain is not reset

[cezaraugusto]

thanks for checking, feedback addressed

[cezaraugusto]

updated. waiting CI

[AlexeyBarabash]

thanks @cezaraugusto , LGTM

[bsclifton]

Great job guys 😄 Let's give this a go on Nightly and then I think we're ready for an uplift back to 1.3

[btlechowski]

LGTM. Verified on Windows and Linux.

Verified clicking Start a new Sync Chain creates sync chain.
Verified clicking Start a new Sync Chain shows Choose Device Type
Verified that Choose Device Type modal is always shown after restart when there is only one device in the chain.
Verified Sync Chain QR Code is shown after choosing Phone/Tablet
Verified Sync Chain Code is shown after choosing Computer
Verified ability to cancel adding device through QR Code
Verified ability to cancel adding device through Code Words
Verified ability to create sync through code words
Verified ability to create sync through QR code
Verified leaving sync chain by clicking Leave sync chain works
Verified leaving sync chain by clicking x next to the device's name works
Verified ability to add additional devices after sync creation

Verification passed on

Brave	1.5.45 Chromium: 79.0.3945.130 (Official Build) nightly (64-bit)
Revision	e22de67c28798d98833a7137c0e22876237fc40a-refs/branch-heads/3945@{#1047}
OS	Windows 7 Service Pack 1 (Build 7601.24544)

Verification passed on

Brave	1.5.45 Chromium: 79.0.3945.130 (Official Build) nightly (64-bit)
Revision	e22de67c28798d98833a7137c0e22876237fc40a-refs/branch-heads/3945@{#1047}
OS	Ubuntu 18.04 LTS

Non critical issues found
Choose Device Type modal not shown after going offline and back online - brave/brave-browser#7854
Unable to close "Choose Device Type" modal brave/brave-browser#2904