Fix SliceProvider threading

Binding a slice that lives in the same process would lead to a
deadlock since we'd sleep the main thread while waiting for it
to decrease the CountdownLatch. Now we're checking if we're
already in the main looper and only posting when necessary.

Test: run code in ag/3082570
Change-Id: Id7c4f9dd8d84bf0e513606cbe07bf87750c567e4