Android Studio found the ADK source dir.
Add a play/pause button to the widget. (Not operational in this commit.)
Main Activity: Load Timer state in onStart() instead of onCreate()

... in case the widget updated it while the activity was stopped (not visible).
Refactor TimerAppWidgetProvider's Timer load/save code.

TODO: Share the code with the main Activity after figuring out whether it
should continue using SharedPreferences (and add listener support) or
switch to a ContentProvider or a bound Service or something.
Implement the widget's start/stop button via an Intent to the WidgetProvider BroadcastReceiver.
The doc says onCreateOptionsMenu() should call super,

so assume the Android Studio template got it wrong and fix it.
Add TimeCounter#toString() to aid debugging.
Change the play/pause button into a smaller play or pause button.