Improve getBattery() algorithm

[domenic]

These changes are mostly editorial or minor correctness improvements. Notable ones:

• Move to the modern definition of secure context
• Move to permissions policy instead of feature policy. Closes #26.
• Avoid "return and continue asynchronously" pattern. It's fine to create the BatteryManager as part of the algorithm (and that in fact matches implementations).
• Always have a battery promise available, instead of having it sometimes be null. This is important for integration with prerendering in https://github.com/jeremyroman/alternate-loading-modes/pull/45.

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