Consolidate all JVMTI jthread decoding.

We had multiple places where we would decode jthreads. This meant that
we did not always check all required constraints before using it. This
CL consolidates all jthread decoding into the
ThreadUtil::GetNativeThread function and adds a helper
ThreadUtil::GetAliveNativeThread function to check the most common
requirements on jthreads passed to JVMTI.

Bug: 66709480
Test: ./test.py --host -j50
Test: cd openjdkjvmti && git grep -W FromManagedThread
Change-Id: Ib6f4bc8510012e0332831bea67e1842a49092917