test(__tests__/getters): adding tests for `getPowerState*`
test(__tests__/getters): adding tests for `getPowerState*`

adds tests for `getPowerState*` functions. Also, adds tests for all supported platforms

re #1108

rebeccahughes/react-native-device-info,rebeccahughes/react-native-device-info,rebeccahughes/react-native-device-info,rebeccahughes/react-native-device-info,rebeccahughes/react-native-device-info,rebeccahughes/react-native-device-info