GC triggering performance optimizations

Copy the correct starting heap size at process init.
Interfaces to set GC/managed heap parameters.

Change-Id: I1080ba60ae64d546c0f35925bc7c931b89c7600b
Add barriers for proper observation sequence

Change-Id: I364837f1c922e972150eef56ba53054d7961326c
Fix -Xjitthreshold.

Previously, we'd always overwrite the user-supplied value because
the architecture-specific default gets set so late.

Bug: https://code.google.com/p/android/issues/detail?id=52017
Change-Id: I469bf9ce599820f5ce3dea346aa8f680deffb0c5