ART: Fix GetThreadInfo

Fix the retrieval of the context classloader. Make sure to have
the field from the Thread class, don't look into a subclass.

Add caching for the field. Also fix the usage restriction of the
function, which is only valid during the Live phase.

Bug: 36654185
Test: ./test.py --host -r -t 924
Change-Id: I5cce41b31c32e59c80bb7c3afa03e8b0975ec54e