Convert ComponentManager into an interface and create a mock

This will help to mock out ComponentManager's functionality for unit
tests in the future.

Change-Id: Ie74c49c6b31b00b0c4d38bf0db715a62a9532bc7
Reviewed-on: https://weave-review.googlesource.com/1785
Reviewed-by: Vitaly Buka <vitalybuka@google.com>