Halve the number of threads in OatFileAssistantBaseTest.RaceToGenerate.

This change halves the runtime of the test.

Bug: 235209274
Test: atest ArtGtestsTargetChroot:OatFileAssistantBaseTest
Change-Id: I4ca4aa40107f5d3ce71b2ab2872e2bce45f99e0d
diff --git a/runtime/oat_file_assistant_test.cc b/runtime/oat_file_assistant_test.cc
index 15182b4..30852fe 100644
--- a/runtime/oat_file_assistant_test.cc
+++ b/runtime/oat_file_assistant_test.cc
@@ -1362,7 +1362,7 @@
   // take a while to generate.
   Copy(GetLibCoreDexFileNames()[0], dex_location);
 
-  const size_t kNumThreads = 32;
+  const size_t kNumThreads = 16;
   Thread* self = Thread::Current();
   ThreadPool thread_pool("Oat file assistant test thread pool", kNumThreads);
   std::vector<std::unique_ptr<RaceGenerateTask>> tasks;