Fix race condition in releasedCallback test

WebViewFunctor::~Handle() does a post() to renderthread for
destruction. So the test needs to fence after the last sync to ensure
the posted message is processed before proceeding.

Fixes: 173156648
Test: hwuiunit --gtest_repeat=10000 --gtest_filter=RenderNode.releasedCallback --gtest_break_on_failure
Change-Id: I71d21cde43db3f689788e1d0685b30f9e6fdec15