Make the ART thread pool implementation use default-sized stacks on Bionic

Bionic's pthreads implementation already gives us threads configured
the way that ART's thread pool wants them to be configured. This
change lets Bionic use cached stacks for ART thread pool workers,
reducing app startup time.

Bug: http://b/158856274
Test: normal ART tests
Change-Id: Ia13a4d4d1bd40191097ab8f4e83473d93e3ca589
1 file changed