Refactors SuggestionSession to make it testable, adds some tests.

The adjustments make the dependencies of SuggestionSession lightweight enough
so the test can more easily inject them:
- SuggestionFactory is an interface, and the necessary methods take in the query
  string when necessary, instead of taking it in as a c'tor paremeter.  this means
  we no longer need to create the factory for each query, just once per session.
- ShortcutRepository is an interface instead of an abstract base class (there is
  no need for the base class anymore anyways since we only have one implementation).
- SuggestionSources implements an interface, SourceLookup, so the sesssion doesn't
  depend directly on the heavy weight logic in SuggestionSources that interacts with
  preferences etc
- Add interface 'DelayedExecutor' to remove direct dependency on the Handler

The test injects a special implementation of Executor and DelayedExecutor so it
can tell what sources have been queried when, allowing us to verify the behavior.

Introducing these new interfaces may seem a bit risky given our memory and speed
constraints, but in practice, this will only add a couple extra objects per session
creation, and I feel a lot better about the stability / maintainability of session
with some tests in place.