Simplify bat_get_media in native ledger

[NejcZdovc]

Current bat_get_media.cc is really all around and it's hard to add any new platform. Because of this we need to simplify it. This file will now only contain shared functionalities and every platform will have their own file with their own functionalities

[LaurenWags]

Verified passed with

Brave	0.65.98 Chromium: 74.0.3729.131 (Official Build) beta(64-bit)
Revision	518a41c1fa7ce1c8bb5e22346e82e42b4d76a96f-refs/branch-heads/3729@{#954}
OS	Mac OS X

• Encountered the following already logged issues: panel not showing twitch publisher info for VOD #3590, twitch.tv instead of publisher name is shown on Brave Panel #3417, Rewards Panel always shows 0% attention on twitch #3409
• Verified test plan from Simplify media structure inside native ledger  brave-core#2167
• Verified additional media related items such as: if the setting for 'Allow Contribution to Videos' is off, no videos were added to ac table. Verified able to tip verified YT and Twitch pubs when AC is off and the expected favicons display in both the Tips list and the Rewards Panel.

Verification passed on

Brave	0.65.107 Chromium: 75.0.3770.38 (Official Build) beta (64-bit)
Revision	3860105745f2b12537da9e9f048f14c3f52ba970-refs/branch-heads/3770@{#618}
OS	Windows 10 OS Version 1803 (Build 17134.523)

• Verified test plan from Simplify media structure inside native ledger  brave-core#2167
• Verified additional media related items such as: if the setting for 'Allow Contribution to Videos' is off, no videos were added to ac table. Verified able to tip verified YT and Twitch pubs when AC is off and the expected favicons display in both the Tips list and the Rewards Panel.

Verification passed on

Brave	0.65.107 Chromium: 75.0.3770.38 (Official Build) beta (64-bit)
Revision	3860105745f2b12537da9e9f048f14c3f52ba970-refs/branch-heads/3770@{#618}
OS	Linux

• Verified test plan from Simplify media structure inside native ledger  brave-core#2167
• Verified additional media related items such as: if the setting for 'Allow Contribution to Videos' is off, no videos were added to ac table. Verified able to tip verified YT and Twitch pubs when AC is off and the expected favicons display in both the Tips list and the Rewards Panel.