Power management setup UI

Copy the power management setup UI from Briar.

changed milestone to %Mailbox: First run

added Sponsor 6 T: feature labels

mentioned in issue #47 (closed)

marked this issue as related to #47 (closed)

added Battery label

assigned to @grote

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added S: in progress label

mentioned in merge request !34 (merged)

added Mailbox label

mentioned in commit cff8f528

closed with merge request !34 (merged)

removed S: in progress label

Examples of Briar's power management setup UI on different devices:

Moto E3 (Android 6.0)

Redmi Note 7 (Android 10 / MIUI 12.5.1)

Huawei Y6P (Android 10 / EMUI 10.1.0)

added Mailbox app label

added R: fixed label