create an espresso companion library

[loganj]

Publish a companion library to help with testing apps that use Flow.

There are a number of ViewMatchers and an IdlingResource (at least) that should be collected into a small but powerful library.

Some sketches:

  /** Matches the first view (DFS) with the given Flow key in its Context. */
  ViewMatcher <T> withKey(Matcher<? super T> key);
  /** Matches the first view (DFS) with the given Flow service in its Context. */
  ViewMatcher <T> withService(String serviceName, Matcher<? super T> service);
​
  /** Convenience for matches(withKey(key)) */
  ViewAssertion <T> hasKey(Matcher<? super T> key);  
​
  /** Pauses state changes. */
  void pauseFlow();
  /** Pauses state changes when the top of the history matches the given key. */
  void <T> pauseOnKey(Matcher<? super T> key);
  /** Unpauses. */
  void resumeFlow();
  /** Waits until there are no queued state changes. */
  void waitUntilFlowIdle();
  /** Resumes immediately, pauses when the top of the history matches the given key. */
  void <T> waitForKey(Matcher<? super T> key);