Swap allocation stacks in pause.

This enables us to collect objects allocated during the GC for both
sticky, partial, and full GC. This also significantly simplifies GC
code. No measured performance impact on benchmarks, but this should
slightly increase sticky GC throughput.

Changed RevokeRosAllocThreadLocalBuffers to happen at most once per
GC. Previously it occured twice if pre-cleaning was enabled.

Renamed HandleDirtyObjectsPhase to PausePhase and enabled it for
non-concurrent GC. This helps reduce duplicated code which was in
both HandleDirtyObjectsPhase for concurrent GC and ReclaimPhase for
non-concurrent GC.

Change-Id: I533414b5c2cd2800f00724418e0ff90e7fdb0252