Ads does not reliably enable when activated from pop-up in rewards settings

[moritzhaller]

Description

Ads does not reliably enable when activated from new opt-in pop-up in Preferences, see screen capture here: https://bravesoftware.slack.com/files/UM5GQEGBV/F01ECRT35V5/screen_recording_2020-11-16_at_09.45.24.mov

Steps to Reproduce

1. start latest Nightly with fresh profile
2. go to rewards settings and click "Start using Brave rewards"

Actual result:

Ads switch not being enabled, but auto-contribute switch being enabled

Expected result:

Ads switch being enabled, but auto-contribute switch being enabled

Reproduces how often:

intermittent on every 2nd to 3rd attempt

Brave version (brave://version info)

Version 1.19.5 Chromium: 87.0.4280.49 (Official Build) nightly (x86_64)

Version/Channel Information:

• Can you reproduce this issue with the current release?
• Can you reproduce this issue with the beta channel?
• Can you reproduce this issue with the nightly channel?

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:

[zenparsing]

I believe that this issue is occurring because the ads service has not registered user pref listeners when the kEnabled ads pref is set to true as a result of the "Enable Rewards" button begin pressed.

• In ads_service_impl.cc, the AdsServiceImpl constructor calls MaybeInitialize, which performs MigrateConfirmationsStateOnFileTaskRunner on a separate task runner.
• When that task is complete, AdsServiceImpl::Initialize is called, which registers pref change listeners.
• If AdsServiceImpl::SetEnabled is called prior to Initialize, then the pref change will not be observed, and consequently rewards_service_->CreateWallet will not be called.

[btlechowski]

QA/Blocked as build > 1.18.68 is needed

[btlechowski]

Verification passed on

Brave	1.18.69 Chromium: 87.0.4280.88 (Official Build) (64-bit)
Revision	89e2380a3e36c3464b5dd1302349b1382549290d-refs/branch-heads/4280@{#1761}
OS	Windows 7 Service Pack 1 (Build 7601.24544)

Verified test plan from brave/brave-core#7276
Verified for Rewards Panel:


Verified for NTP:


Verified for brave://rewards

Checked the logs for any errors
Verified on production and staging

---

Verification passed on

Brave	1.18.70 Chromium: 87.0.4280.101 (Official Build) (64-bit)
Revision	9407c80213cda69c2b7abcb4fa8e3f74488f4956-refs/branch-heads/4280@{#1807}
OS	Windows 7 Service Pack 1 (Build 7601.24544)

Verified test plan from brave/brave-core#7276
Verified for Rewards Panel:

Verified for NTP:

Verified for brave://rewards

Checked the logs for any errors