NTP SI - Previous image is visible after changing from supported region to un-supported region

[simonhong]

After changing to un supported region from supported region, ntp sponsor image should not be visible.

Repro step

1. launching brave with clean profile with US region
2. Turn on the New Tab Page Branded Wallpapers only and re-launch
3. Check sponsorship image is visible
4. change OS region to CA and re-launch
5. Check sponsorship image is not visible

[btlechowski]

Verification passed on

Brave	1.3.108 Chromium: 79.0.3945.130 (Official Build) beta (64-bit)
Revision	e22de67c28798d98833a7137c0e22876237fc40a-refs/branch-heads/3945@{#1047}
OS	Ubuntu 18.04 LTS

Verified STR from the description. The sponsored images were not shown for Canada.
Used following command lines to trigger region change.
LANG=en_US.UTF-8 LANGUAGE=en_US /usr/bin/brave-browser-beta
LANG=en_CA.UTF-8 LANGUAGE=en_CA /usr/bin/brave-browser-beta

Verification passed on

Brave	1.3.109 Chromium: 79.0.3945.130 (Official Build) beta (64-bit)
Revision	e22de67c28798d98833a7137c0e22876237fc40a-refs/branch-heads/3945@{#1047}
OS	Windows 10 OS Version 1803 (Build 17134.1006)

• Verified STR from the description. The sponsored images were not shown for Canada.

Verification PASSED on macOS 10.15.3 x64 using the following build:

Brave	1.3.108 Chromium: 79.0.3945.130 (Official Build) beta (64-bit)
Revision	e22de67c28798d98833a7137c0e22876237fc40a-refs/branch-heads/3945@{#1047}
OS	macOS Version 10.15.3 (Build 19D76)

• Verified the STR outlined under NTP SI - Previous image is visible after changing from supported region to un-supported region #7946 (comment)
• Checked Canada, UK, Germany, France