Avoid a long alloc space lock contention during sticky GC sweep.
Avoid a long alloc space lock contention during sticky GC sweep.

The lock contention logging systrace support (b/9986464) indicates
that under the Ritz allocation benchmark the alloc space lock contends
rather long (up to ~10 ms on Nexus 4) in a single lump during each
sticky GC sweep. This is because all the dead objects in the
allocation stack are freed in one FreeList() call. While the lock
contention isn't a GC pause and may only be an issue with a
high-allocation application like Ritz, it'd stall any application
threads that attempt to allocate during the contention and would be
bad for jank. I think it's better to free the objects incrementally in
chunks and interleave the GC thread and application threads.

This change does not speed up or slow down the Ritz allocation
benchmark noticeably.

Bug: 9986464
Change-Id: I24ea805456c0009752991f7d3eb031f6f57145fc

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