Prevent battery depletion/network roaming cost on Android

[choksi81]

We'll want some additional safety nets for donors running on Android.

1. Let's not deplete the battery once its level goes below a threshold. I for one get a warning message when the charge drops below 15%, so this might be a good point when to pause Seattle.
2. We should also pause if we detect we're on a "foreign" (whatever this means) cellular networks, because roaming plus data plan usually equals high cost for the poor donor.

We could put this functionality right into the Seattle APK; another way were to use the new Sensor interface and come up with separate little apps that check resources and send signals to the node manager. We could even suggest these apps at install time, or declare them dependencies of the main package.