Add an option to set solid color background for NTP

[sangwoo108]

sub task of #15252 . Overall design also can be found there.

Spec:

• Users can choose a solid color for NTP background
• This will be applied only to the current local machine.
• When Users choose another option like custom background and brave background, the background should be changes to it.

Open discussion:

• What about sponsored images? The official version seems to show SI once per 4 new tabs. Need to check if this works same. Currently, the same SI policy is applied to solid color bg too.

[MadhaviSeelam]

Verification PASSED using

Brave | 1.44.73 Chromium: 105.0.5195.68 (Official Build) beta (64-bit)
-- | --
Revision | ad13e82529051bac6a0e65f455e6d7a1e5fd7938-refs/branch-heads/5195@{#903}
OS | Windows 11 Version 21H2 (Build 22000.856)

1. install 1.44.73
2. launch Brave
3. click Skip welcome tour
4. click Customize bottom right on the NTP
5. click Solid colors
6. select a color
7. confirmed NTP background is set to selected color
8. select another color and NTP background is set to selected color
9. verify Brave News is peeked as expected
10. click Brave Backgrounds to return to original background

step 4	step 5	step 6-7	step8-9	Step 10

[stephendonner]

Verified PASSED using

Brave	1.44.79 Chromium: 105.0.5195.102 (Official Build) beta (x86_64)
Revision	4c16f5ffcc2da70ee2600d5db77bed423ac03a5a-refs/branch-heads/5195_55@{#4}
OS	macOS Version 13.0 (Build 22A5331f)

Steps:

1. install 1.44.73
2. launch Brave
3. click Skip welcome tour
4. click Customize bottom right on the NTP
5. click Solid colors
6. select a color
7. confirmed NTP background is set to selected color
8. select another color and NTP background is set to selected color
9. verify Brave News is peeked as expected
10. click Brave Backgrounds to return to original background

step 4	step 5	step 6-7	step 8-9	step 10

[stephendonner]

Verified PASSED using

Brave	1.44.88 Chromium: 105.0.5195.136 (Official Build) beta (64-bit)
Revision	872774b783d0e674186a3adcd2f92e7aa22a219c-refs/branch-heads/5195_124@{#4}
OS	Linux

Steps:

1. install 1.44.88
2. launch Brave
3. click Skip welcome tour
4. click Customize bottom right on the NTP
5. click Solid colors
6. select a color
7. confirmed NTP background is set to selected color
8. select another color and NTP background is set to selected color
9. verify Brave News is peeked as expected
10. click Brave Backgrounds to return to original background

example	example	example	example	example	example