Disable parallel GC by default

Not using parallel GC seems to reduce avg pauses by ~0.1s on
EvaluateAndApplyChanges. Avoiding creating the thread pool should
help app launch slightly and reduce memory ussage.

Change-Id: Iebec2a17701c76e4145b41d7c0b4f6dd17806efa
diff --git a/runtime/gc/collector/mark_sweep.cc b/runtime/gc/collector/mark_sweep.cc
index bb8d876..e65d1f1 100644
--- a/runtime/gc/collector/mark_sweep.cc
+++ b/runtime/gc/collector/mark_sweep.cc
@@ -740,11 +740,7 @@
   if (heap_->GetThreadPool() == nullptr || !heap_->CareAboutPauseTimes()) {
     return 1;
   }
-  if (paused) {
-    return heap_->GetParallelGCThreadCount() + 1;
-  } else {
-    return heap_->GetConcGCThreadCount() + 1;
-  }
+  return (paused ? heap_->GetParallelGCThreadCount() : heap_->GetConcGCThreadCount()) + 1;
 }
 
 void MarkSweep::ScanGrayObjects(bool paused, uint8_t minimum_age) {