Change ProcessReferences to not use RecursiveMarkObject.

Calling ProcessMarkStack in RecursiveMarkObject caused a lot of
overhead due to timing logger splits. Changed the logic to be the
same as prior to the reference queue refactoring which involves
calling process mark stack after preserving soft references and
enqueueing finalizer references.

FinalizingGC longest pause is reduced by around 1/2 down to ~300ms.
Benchmark score ~400000 -> ~600000.

Also changed the timing logger splits in the GC to have (Paused) if
the split is a paused part of the GC.

Bug: 12129382

Change-Id: I7476d4f23670b19d70738e2fd48e37ec2f57e9f4