Multiple provider / widget support

[noxasch]

Currently multiple widget of the same provider works just fine. This is to support multiple widget types on Android.

[noxasch]

What need to be done:

• To support un-configurable widget, we need to expose widget onUpdate as a callback that trigger an isolate in the background since widget provider can't communicate with mainActivity.

• To support multiple configurable widget, we can't use CONFIGURE_WIDGET intent as it doesn't include the widget layout. Instead implement a default onClick callback on the widget during the first onUpdate/onEnabled callback. This callback will then return the widgetId and layout name to the configuration screen/activity.

[noxasch]

Another option to try is to use another activity similar to how we handle this in Android. This activity should return the appropriate widget id and layout name when flutter engine is ready.

NOTE: not sure if using multiple activity will have any effect on the flutter app since each activity call the same flutter app.

[noxasch]

Update:

• expose layoutId and layout name is planned in v0.4.0 in both configureWidget and updateWidget

with this implementation theoretically we can support widget with different layout without much works.

[noxasch]

to be verify in v0.4.0