frontend: Change static AsyncDataProvider to singleton

Replace all references to static AsyncDataProvider methods by
AsyncDataProvider.getInstance() and all methods change to instance
methods.

This also enables mocking the AsyncDataProvider for test purposes by
using the new AsyncDataProvider#setInstance() static method.

Change-Id: I3bfc604834856e8240703bb8bf64e9a40c3b8c0d
Signed-off-by: Martin Betak <mbetak@redhat.com>