preserve "Show Sponsored Images" off setting when "Show Background Image" is toggled on

[rebron]

Description

On the New Tab Page, when going to dashboard settings and disabling "Show Sponsored Images" and "Show Background Image", "Show Sponsored Images" will get reenabled when selecting "Show Background Image".

Only "Show Background Image" should be toggled on while "Show Sponsored Images" should continue to be toggled off.

Test plan / Steps to Reproduce

1. Go to brave://newtab -> and select dashboard setting
2. Toggle OFF both "Show Background Image" and "Show Sponsored Images"
3. Toggle ON "Show Background Image".
4. Notice that "Show Sponsored Images" is also toggled ON. This pref should stay off.

[rebron]

cc: @petemill

[GeetaSarvadnya]

Verification passed on

Brave	1.7.67 Chromium: 80.0.3987.149 (Official Build) beta (64-bit)
Revision	5f4eb224680e5d7dca88504586e9fd951840cac6-refs/branch-heads/3987_137@{#16}
OS	Windows 10 OS Version 1803 (Build 17134.1006)

• Verified the test plan from NTP: Do not auto opt-in sponsored images after re-enabling backgrounds brave-core#4875
• Verified in both clean and upgraded profile, toggled OFF both "Show Background Image" and "Show Sponsored Images" in 1.5.x and then upgraded to 1.7.x If we trun ON "Show Background Image" in 1.7.x "Show Sponsored Images" still remained off.
• Encountered "Show Sponsored Images" toggle under "Dashboard Settings" should look the same as others #8057
  

Verification passed on

Brave	1.7.70 Chromium: 80.0.3987.149 (Official Build) dev (64-bit)
Revision	5f4eb224680e5d7dca88504586e9fd951840cac6-refs/branch-heads/3987_137@{#16}
OS	Ubuntu 18.04 LTS

• Verified the test plan from NTP: Do not auto opt-in sponsored images after re-enabling backgrounds brave-core#4875

---

Verified passed with

Brave	1.7.71 Chromium: 80.0.3987.149 (Official Build) dev (64-bit)
Revision	5f4eb224680e5d7dca88504586e9fd951840cac6-refs/branch-heads/3987_137@{#16}
OS	macOS Version 10.14.6 (Build 18G3020)

• Verified test plan from NTP: Do not auto opt-in sponsored images after re-enabling backgrounds brave-core#4875
• Verified STR from description
• Encountered "Show Sponsored Images" toggle under "Dashboard Settings" should look the same as others #8057