Enables background playback on Android on release channel for 20 percent of users

[SergeyZhukovsky]

Resolves #494

[kjozwiak]

LGTM 👍 Getting this merged into main so QA can verify the above on staging before uplifting #496.

[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 | 1.46.146 Chromium: 108.0.5359.128 (Official Build) (64-bit)
--- | ---
Revision | 1cd27afdb8e5d057070c0961e04c490d2aca1aa0-refs/branch-heads/5359@{#1185}
OS | Android 13; Build/TQ1A.221205.011

BgPlaybackStudy:Enabled

• launched 1.46.146 Chromium: 108.0.5359.128 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

Before restarting Brave and pulling the studies

Example	Example

After restarting Brave and pulling the studies

Example	Example	Example

BgPlaybackStudy:Default

• launched 1.46.146 Chromium: 108.0.5359.128 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

Before restarting Brave and pulling the studies

Example	Example

After restarting Brave and pulling the studies

Example	Example