Brave Ads may not be converted for duration of observation window

[tmancey]

Description

Brave Ads may not be converted for duration of observation window depending on user activity and ads history.

Steps to Reproduce

1. View an ad notification (which has a conversion)
2. Wait for the ad notification to time out
3. Trigger 700 thumbs up events on the 7-days ad history screen (use staging so you do not run out of unblinded tokens)
4. Trigger the conversion from step 1

Actual result:

Ad is not converted due to the ads history only storing 700 entries (enough for ~7 days depending on how many events are triggered)

Expected result:

Ad should be converted for the duration of the observation window

Reproduces how often:

Easily reproduced

Brave version (brave://version info)

Version/Channel Information:

• Can you reproduce this issue with the current release? Yes
• 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? N/A
• Does the issue resolve itself when disabling Brave Rewards? N/A
• Is the issue reproducible on the latest version of Chrome? N/A

Miscellaneous Information:

[tmancey]

Closing as issue was resolved by refactoring #12486 where we moved ad event history from client.json to an ad_events database table

[btlechowski]

Verification passed on

Brave	1.18.66 Chromium: 87.0.4280.67 (Official Build) dev (64-bit)
Revision	0e5d92df40086cf0050c00f87b11da1b14580930-refs/branch-heads/4280@{#1441}
OS	Windows 7 Service Pack 1 (Build 7601.24544)

Verified test plan from the description.
Verified that after 700 events on 7-day history page, the ad was converted.