[asan][fuchsia] set current thread before reading thread state

When enabling stack use-after-free detection, we discovered that we read
the thread ID on the main thread while it is still set to 2^24-1.

This patch moves our call to AsanThread::Init() out of CreateAsanThread,
so that we can call SetCurrentThread first on the main thread.

Reviewed By: mcgrathr

Differential Revision: https://reviews.llvm.org/D89606