Add a setting to control whether Briar uses a wake lock
Briar holds a wake lock while Tor is running. @gdt asked for a setting to disable the wake lock, which would mean messages wouldn't be synced while the device was sleeping, but battery usage would be reduced.
If no other app is holding a wake lock, the device typically sleeps a few seconds after the screen turns off. We probably don't want to lose connectivity immediately, and it would be good if we could prepare for sleep so Tor doesn't panic when it wakes up. We might consider holding a wake lock for a few minutes after the screen turns off, then cleanly going offline and releasing the wake lock. Then we can listen for ACTION_SCREEN_ON to reacquire the wake lock and go online.
Alternatively, if we want to be really hardcore about saving battery at the expense of connectivity, we can go offline when the screen turns off. In that case no wake lock would be needed.