Make StopWatchingFileDescriptor deterministic.

The AsyncManager allows for registering and unregistering watchers for
sockets. Previously it was possible to receive a callback even though
unregister was called from a separate thread.

This makes it very difficult to properly manage object lifetime for an
object that took a dependency on a callback, as a callback could arrive
while the object was destroyed.

We now introduce a recursive_mutex to protect the subscriptions. This
guarantees:

- We will block unsubscription until all relevant callbacks have
  finished.
- We can still stop watching from the callback thread.

This guarantees that we will never get a callback after unsubscription.

Includes 2 unit tests:

1. Guarantee that we do not deadlock in unsubscription from a callback.
2. Attempt to validate that a callback never happens after unsubscribe.

The 2nd test will fail when the recursive mutex is removed.

Test: 2 additional unit tests.
Change-Id: I2b9cdb0c31f88e67bbaca0667bd8822f69096995