Only allocate collectors if we may use them

I believe most of the savings come from not allocating the concurrent
copying 8MB MarkQueue.

Before AOSP hh native PSS:
    72653 kB: Native
    72998 kB: Native
    72882 kB: Native

After AOSP hh native PSS:
    64823 kB: Native
    65986 kB: Native
    64219 kB: Native

Bug: 17643507
Change-Id: Ic5a8e753beca36142c1bf36be6311051f7c78e47
2 files changed