Follow-up to #981.

OIDExternalUserAgentIOSCustomBrowser called UIApplication.sharedApplication directly, so -presentExternalUserAgentRequest:session: was untestable. (The iOS unit test target has no host app, so the completion handler will never fire)

Functional-programming-enjoyer added DI so that the behaviour in #981 is testable:

• OIDExternalUserAgentIOSCustomBrowserURLOpener, a two-method protocol to which UIApplication can conform.
• A URLOpener property and a new designated initializer initWithURLTransformation:canOpenURLScheme:appStoreURL:URLOpener:.
• UnitTests/OIDExternalUserAgentIOSCustomBrowserTests.m (iOS only):
  • presenting returns YES and opens the transformed URL without consulting canOpenURL: for the request URL (this is what Custom browser user-agent fails the flow with "Unable to open Safari." while the browser opens (it returns the result of canOpenURL:) #980 fixed).
  • an open failure fails the session exactly once with OIDErrorCodeSafariOpenError.
  • an open success does not fail the session.
  • a nil transformed URL returns NO without making any calls to UIApplication.
  • OIDAuthorizationSession reports a failure only once, even if failExternalUserAgentFlowWithError: is called twice. Fix custom browser user-agent failing the flow when canOpenURL: returns NO #981 relies on this, since the failure now arrives from an asynchronous completion handler after the session may already have been cancelled or finished.