Import button should be disabled until user selection, in new onboarding experience

[stephendonner]

Description

Import button should be disabled until user selection, in new onboarding experience

Steps to Reproduce

1. install 1.48.68
2. launch Brave
3. clip Skip on the 1st onboarding page
4. play with the Import button on the next page

Actual result:

The Import button is enabled, without a selection.

Screencast:

Expected result:

The Import button should be disabled until the user has made a selection

Reproduces how often:

100%

Brave version (brave://version info)

Brave	1.48.68 Chromium: 109.0.5414.46 (Official Build) nightly (x86_64)
Revision	6e36b77363ef3febbe792af680fa1367993ddcf0-refs/branch-heads/5414@{#709}
OS	macOS Version 11.7.2 (Build 20G1020)

Version/Channel Information:

• Can you reproduce this issue with the current release? No
• Can you reproduce this issue with the beta channel? Yes
• Can you reproduce this issue with the nightly channel? Yes

Other Additional Information:

• Does the issue resolve itself when disabling Brave Shields?
• Does the issue resolve itself when disabling Brave Rewards?
• Is the issue reproducible on the latest version of Chrome?

Miscellaneous Information:

/cc @btlechowski @nullhook @rebron