Enables bg playback by default for 20 percent users on release channel

[SergeyZhukovsky]

Resolves #525

[kjozwiak]

LGTM 👍 Merging into main so QA can verify on staging before merging #527.

[kjozwiak]

Went through the STR/Cases mentioned via brave/brave-core#10408 (comment) and ensured that Brave was using https://variations.bravesoftware.com/seed which is the staging URL.

Release channel

Verification PASSED on Pixel 6 running Android 13 using the following build(s):

Brave | 1.48.171 Chromium: 110.0.5481.177 (Official Build) (32-bit)
--- | ---
Revision | f34f7ab2d4ca4ad498ef42aeba4f4eb2c1392d63-refs/branch-heads/5481@{#1239}
OS | Android 13; Build/TQ1A.230205.002

BgPlaybackStudy:Enabled

• launched 1.48.171 Chromium: 110.0.5481.177 and ensured that Background video playback was disabled
• restarted Brave and ensured that BgPlaybackStudy:Enabled via brave://version as the client used the pulled Griffin studies
• ensured that Background video playback was enabled via Hamburger Menu -> Settings
• ensured that videos on YT are still playing when Brave is moved to the background and other applications are being used
• ensured that videos on YT are still playing when the phone has been locked
• ensured that the video that's being played is appearing the via Notification area while the phone is locked
• ensured that can't disable video background playback when enabled via BgPlaybackStudy brave-browser#27550 doesn't occur

Before restarting Brave and pulling the studies

Example	Example

After restarting Brave and pulling the studies

Example	Example	Example	Example

BgPlaybackStudy:Default

• launched 1.48.171 Chromium: 110.0.5481.177 and ensured that Background video playback was disabled
• restarted Brave and ensured that BgPlaybackStudy:Default via brave://version as the client used the pulled Griffin studies
• ensured that Background video playback was disabled via Hamburger Menu -> Settings
• ensured that can't disable video background playback when enabled via BgPlaybackStudy brave-browser#27550 doesn't occur

Before restarting Brave and pulling the studies

Example	Example

After restarting Brave and pulling the studies

Example	Example