update arrival date value for ad grant UI - follow up to #15005

[LaurenWags]

Description

Found while testing #15005, the "arriving in X days" is sometimes rounding up, which can be confusing when comparing it to the displayed "Next payment date".

See https://bravesoftware.slack.com/archives/CB0KCRCQN/p1625153601283200 for additional context.

Steps to Reproduce

1. Have ad tokens cashed in for previous month.
2. On the 1st of the next month, go to brave://rewards or NTP widget.
3. View "arriving in X days" message.

Actual result:

Message says my ad grant is arriving in 5 days (I think that means July 6th, today + 5 days)

Example	Example

Expected result:

I was expecting it to say 4 days since payment date is listed as July 5 (today + 4 days).

Reproduces how often:

easily

Brave version (brave://version info)

Brave	1.27.87 Chromium: 91.0.4472.124 (Official Build) beta (x86_64)
Revision	7345a6d1bfcaff81162a957e9b7d52649fe2ac38-refs/branch-heads/4472_114@{#6}
OS	macOS Version 10.15.7 (Build 19H1217)

Version/Channel Information:

• Can you reproduce this issue with the current release? n/a
• 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 @zenparsing @Miyayes

[stephendonner]

Verified PASSED using the inline testplan with build

Brave	1.28.96 Chromium: 92.0.4515.115 (Official Build) beta (x86_64)
Revision	48cb2f4029b84b003719740a6cf9ca73f374a857-refs/branch-heads/4515_105@{#4}
OS	macOS Version 11.5.1 (Build 20G80)

Steps:

1. have ad tokens cashed in for previous month
2. on the 1st of the next month, opened a new tab page with Brave Rewards widget and brave://rewards
3. noted their "arriving in X days" messages

Confirmed +0.06 BAT arriving in 6 days is correct (matches Next payment date in brave://rewards).

brave://rewards	Brave Rewards widget