5th background image (yellow) isn't showing up on tip banner

[LaurenWags]

Description

Found while testing #11393

Per figma design in above issue, there should be 5 randomly used background images. 4/5 are showing up but the 5th (yellow) is not.

Steps to Reproduce

1. Have Rewards enabled
2. Open a bunch of sites w/o custom tip banners and open tip banner on each

Actual result:

the various background colors are used randomly, but yellow doesn't show up

Expected result:

yellow should be used occasionally

Reproduces how often:

easily

Brave version (brave://version info)

Brave	1.17.63 Chromium: 87.0.4280.49 (Official Build) unknown (x86_64)
Revision	f77f85899646b42a1d3c8ff36794e00becab9171-refs/branch-heads/4280@{#1115}
OS	macOS Version 10.14.6 (Build 18G6032)

Version/Channel Information:

• Can you reproduce this issue with the current release? n/a 1.16.x
• Can you reproduce this issue with the beta channel? yes 1.17.x
• Can you reproduce this issue with the nightly channel? unsure

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 @zenparsing @NejcZdovc

[GeetaSarvadnya]

Verification passed on

Brave | 1.19.55 Chromium: 87.0.4280.101 (Official Build) dev (64-bit)
-- | --
Revision | 9407c80213cda69c2b7abcb4fa8e3f74488f4956-refs/branch-heads/4280@{#1807}
OS | Windows 10 OS Version 2004 (Build 19041.630)

• Verified the STR from the issue
  
  
  
  
  

---

Verified passed with

Brave | 1.19.73 Chromium: 87.0.4280.101 (Official Build) dev (x86_64)
-- | --
Revision | 9407c80213cda69c2b7abcb4fa8e3f74488f4956-refs/branch-heads/4280@{#1807}
OS | macOS Version 10.15.7 (Build 19H15)

Verified STR from description and test plan from brave/brave-core#7215
Confirmed all 5 tip banner backgrounds are used:

---

Verification passed on

Brave	1.19.73 Chromium: 87.0.4280.101 (Official Build) dev (64-bit)
Revision	9407c80213cda69c2b7abcb4fa8e3f74488f4956-refs/branch-heads/4280@{#1807}
OS	Ubuntu 18.04 LTS

Yellow

Red

Blue

Purple

Green