Fix stale remembered sets error.

We were forgetting to remove the remembered set in transition to
background. This resulted in remembered sets being added for
spaces which no longer existed. This finally caused an error when
a new space happened to have the same address as the old space,
resulting in a CHECK failure.

Also tuned the number of ParallelGC to prevent spurrious failures
and removed the ParallelGC from broken tests in the make file.

Bug: 16532086
Bug: 16406852

Change-Id: I00bbcbd7daa03c867732d165be62b72e6c43bce1

(cherry picked from c5a8347a)