Ensure the ActionProxyReceiver runs as the system user

- Secondary users have a SysUI process but no status bar, so we need to
  actually call executeRunnableDismissingKeyguard() from the system user.
  (The actual call to start the proxied activity will still be handled as
  the current user though)

Bug: 72459081
Test: Switch to secondary user, take screenshot and then try to share/edit
Change-Id: Id7bcd6f9090412902d90566de074d1c2f235b8d6