Migrate bat-native-ads to brave-core

[tmancey]

Test plan

See brave/brave-core#1871

[tmancey]

Test Plan: Confirm Ads work as expected, however this has been tested in other tickets so how much needs testing would depend on other tickets

[GeetaSarvadnya]

Verification passed on

Brave	0.63.40 Chromium: 74.0.3729.61 (Official Build) beta (64-bit)
Revision	5df2c8936783bd7575987e45d72a92fcf528496b-refs/branch-heads/3729@{#645}
OS	Windows 10 OS Build 17134.523

• Verified "View", "click", "landed" and "dismiss" status in Confirmations.json
• Verified Ads received and current earnings displayed properly in Ads panel in brave://rewards page
• Encountered Confirmation type dismiss is not recorded in confirmations.json file #4023, Unable to open Ads from Notification center #4102 and Brave Ads target URLs that redirect to a different hostname are not sustained #4026

Verification passed on both

Brave	0.63.40 Chromium: 74.0.3729.61 (Official Build) beta(64-bit)
Revision	5df2c8936783bd7575987e45d72a92fcf528496b-refs/branch-heads/3729@{#645}
OS	Linux

Brave	0.64.47 Chromium: 74.0.3729.61 (Official Build) dev(64-bit)
Revision	5df2c8936783bd7575987e45d72a92fcf528496b-refs/branch-heads/3729@{#645}
OS	Linux

• Verified View, Click, Landed and Dismiss transactions are logged in confirmations.json
• Verefied the total ads viewed matches the count in rewards and the payout for ads matches the transaction type

Verified passed with

Brave	0.63.44 Chromium: 74.0.3729.75 (Official Build) beta(64-bit)
Revision	fdb7915642fef8cf997beac2554709d148e3c187-refs/branch-heads/3729@{#754}
OS	Mac OS X

• Ran thru Confirmations Test Plan (separate google doc)
• Verified View, Click, Landed and Dismiss transactions are logged in confirmations.json
• Verified the total ads viewed matches the count in rewards and the payout for ads matches the transaction type