Children of Sync device type modal should use the same modal

[cezaraugusto]

Test plan

See brave/brave-core#1193

Description

carried over from #2500

Device type modal opens a new modal every time user choose either desktop or mobile versions. This is bad UI and should be fixed using only 1 modal with dynamic content.

Test Plan:

1. Open brave://sync
2. Click "Start a new sync chain"
3. Click "Phone/Tablet"
4. Click "Enter code words instead..."
5. Click "Use my camera instead..."
6. Click "ok"

What happens: You have to click "ok" several times before modal is closed
What should happen: Clicking "ok" should close all dialogs

[cezaraugusto]

setting this as QA/no since this is expected behavior and didn't add new features

[btlechowski]

Changed the label to QA/Yes after talking to @kjozwiak.

[btlechowski]

Verification passed on

Brave	0.59.20 Chromium: 72.0.3626.28 (Official Build) beta (64-bit)
Revision	997b1040b63bac324e815797ba52be0cd8f616ed-refs/branch-heads/3626@{#461}
OS	Windows 7

Used test plan from brave/brave-core#1193. For convenience I included the test plan below:

Fix Children of Sync device type modal should use the same modal

Test plan
1. Clear profile
2. Click "Start a New Sync Chain"
3. Click "Tablet or Phone"
4. Keep toggling between "View Sync Code" and "View QR Code"
5. Trying to close it closes them all at once

Repeat the same process with a sync chain set up by clicking "Add New Device" in the devices table screen.

Verification passed on Ubuntu 17.10

Brave	0.59.20 Chromium: 72.0.3626.28 (Official Build) beta (64-bit)
Revision	997b1040b63bac324e815797ba52be0cd8f616ed-refs/branch-heads/3626@{#461}
OS	Linux

Verified passed with

Brave	0.59.21 Chromium: 72.0.3626.28 (Official Build) beta(64-bit)
Revision	997b1040b63bac324e815797ba52be0cd8f616ed-refs/branch-heads/3626@{#461}
OS	Mac OS X

• Verified test plan for this issue from fix multiple sync issues brave-core#1193