Goal

Improve the startup threading behavior of the SDK to reduce the chances of deadlocks & ANRs.

Design

Previously the SDK startup included several calls which moved work to a background thread, and then used Future.get() to block until the work was complete. This was done to keep I/O operations (and similar) off the main thread, but introduced the cross-thread overheads without any direct benefit (all the cost of a thread, without actually doing anything in parallel).

This PR introduces a new model based around a Provider interface, which is similar to FutureTask in concept. The primary implementation is RunnableProvider which has a very primitive work-stealing ability to ensure that Providers do not block each other (avoiding possible deadlocks where tasks are blocked waiting for tasks on the same queue) unless required. That is: a Provider will typically try and "do" another providers work (unless that provider has already done, or is busy doing said work).

As far as possible the new model does not block until the data from a Provider is actually required by a non-provider (such as the NDK module, or a crash handler). This should allow better use of the threads available during startup.

Testing

Modified the existing unit tests to handle the changes to the models.