Add soft reference pre processing.

Add soft reference pre processing.

Soft reference pre-processing does soft reference preservation with
mutators running. After this is done, it does another pass with
mutators paused in the ProcessReference code. This helps lower pauses
since most preserved soft references have their referents recursive
marked outside the pause.

Changed ergonomics to have non sticky collectors always clear the
soft references.

Maps pauses ~10ms -> ~3ms on Nexus 4.

Bug: 13421927

Change-Id: I1370f7bb6934034869aa5afca0c377876267aa8e