sdk: Kill app: Actually kill app

* Calling forceStopPackage with USER_CURRENT won't kill the app when
  started e.g. via "Shelter" work profile
* Find the uid of the current foreground app and pass that one
  instead of UserHandle.USER_CURRENT
* Note: UserHandle.USER_ALL would also kill the app, but all instances
  of it - which we don't want

Test:
- Go to Settings->System->Buttons and enable "Kill foreground app"
  as action for "Long edge swipe" - test it for PlayStore
- Install Shelter and setup your profile
- Open PlayStore from work profile
- Long swipe the edge:
  - Before: Toast appears, app stays open
  - After: Toast appears, app is actually closed

- Additionally: Open PlayStore from Work and Non-Work profile
- Long swipe edge for just work profile one and switch to own profile
  instance: App is not killed (expected, we just killed work instance)

Change-Id: I82154a6c021527013269f7ee2d1c46fedc14bf7c