* Adds an option to time initialization of FIRAuth instance in the sample app.
* Changes deployment target from 10.3 to 8.0 for Auth samples.
* Fixes some Xcode warnings in the Auth sample app.
* Moves most of `FIRAuth` initialization to the work thread.
* Fixes a typo.
* Fixes accessing of public `currentUser` method from internal method.
* Fixes tests for macOS.
* Addresses review comments.
Xiangtian Dai