FIRApp refactoring - thread safety

[maksymmalyhin]

We need to consider a cleaner way to support thread safety of FIRApp. It was brought up as a result of #2639 (see related issue, discussions and history)

Right now @synchronized(self) is used for this purpose. The main disadvantages of this approach is:

• it requires extra attention from a developer modifying the class. It is easy to miss when new logic is implemented. In fact, the original issue happened juts because some places where missed

In addition:

• it may lead to unintentional blocking of main thread by a background task
• it is possible to introduce a deadlock

[ryanwilson]

Will likely wait until there's an actual issue reported before we go in and start changing the threading mechanism for Core.