Adding support for passing arbitrary extras when requesting to pin a widget.

Also defining an extra constant for widget preview which can be used by
developers to provide a snapshot of the widget with the pin request

Bug: 35811129
Test: All exisiting tests passing
 for f in 1 2 3 4 5 6 7 8 9 10; do \
   adb shell am instrument -e class com.android.server.pm.ShortcutManagerTest$f \
      -w com.android.frameworks.servicestests/android.support.test.runner.AndroidJUnitRunner; \
        done;

 adb shell am instrument -e class com.android.server.appwidget.AppWidgetServiceImplTest \
   -w com.android.frameworks.servicestests/android.support.test.runner.AndroidJUnitRunner

Change-Id: Id854bd28468a5bf0416ff1a1b19c44d850016f32