Make agent threads retain their thread names

We were previously ignoring the thread-names given to agent threads.
This could make debugging issues difficult as every thread was called
"JVMTI Agent thread". Change the RunAgentThread code so instead we
will use the same thread-name as the java thread.

Bug: 68839405
Test: ./test.py --host -j50
Change-Id: Ifdd58e4eebc1877bde0ba49d4fbedbd3935b312f