Have window.create() call window.show().

E.g. if a notification is fired the app will be in the background, but then
that notification calls window.create(), we want the activity to show.
chrome.alarms: Make it more robust and prevent onLaunched when onAlarm is the cause of the Activity starting up

More robust by:
- Eliminated race condition with alarms firing during start-up
- Don't use evaluateJavaScript()
- Remove alarm entry before calling callback (exceptions caused alarms to re-fire)