Wrong grant type in the response to Grants::FetchGrants is resolved as Ads grant and presented on UI so.

[gdregalo]

Description

If server by mistake sends unexpected value in grant.type in response to FetchGrants, then it will be resolved as ads grant and presented to a user so.
https://github.com/brave/brave-core/blob/master/components/brave_rewards/browser/rewards_notification_service_impl.cc#L337.
It would be good if we have a check for acceptable grants types when parsing json.

Steps to Reproduce

1. Modify grant type when parsing json response to FetchGrants.

Actual result:

The Ads grants is presented on UI.
Observable on Android.

Expected result:

Check for acceptable grants types when parsing json response. Ignore corrupted response or log it and add to statistics sent back to Brave.

Reproduces how often:

Brave version (brave://version info)

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 dev 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:

[NejcZdovc]

this one should be addressed with brave/brave-core#3918