Improve internal ExPlat usage and cache control

[ThomazFB]

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Our current configuration of the ExPlat feature involves starting the first setup by requesting variations of the A/B tests we want to cover when the user opens the app and then operating the experiment store directly where we want to execute the A/B scenario and get the cached assignments on FluxC. This is not the usage designed by the ExPlat class.

To fix this, we need to do the following:

• Adjust the ExPlat class in the Tracks repo to support a start command and fetch all configured assignments without requesting each variation individually.
• Adjust when we should clear the cached assignments to avoid outdated or incorrect variation values.
• Avoid operating the experiment store directly.

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