Halve the number of threads in OatFileAssistantBaseTest.RaceToGenerate. am: aed0183b0d am: 2bbb0e1319 am: 7600c99448 am: 9bdb67ce1a

Original change: https://android-review.googlesource.com/c/platform/art/+/2117752

Change-Id: Ic9b755ae4ba6d10a121e59229534a1965f6e45b8
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>