SF: Do not block the main thread on screenshots

The destructor of the future returned by std::async blocks until the
async operation completes, which defeats the purpose of spinning up
a thread to wait on the RenderEngine future. Instead, defer the wait
back to the Binder thread.

Bug: 232436803
Test: Perfetto
Change-Id: I7260002741a0174d8960894d490079757dc4bb5b