graphics: set maxThreads to 4 for the allocator

This should help buffer allocation performance.  We choose 4 because
that is the number of binder threads in SurfaceFlinger.

Bug: 38243574
Test: manual
Change-Id: I704aacb271fb7fd6a91c9231ab8a5256531eebc9
diff --git a/graphics/allocator/2.0/default/service.cpp b/graphics/allocator/2.0/default/service.cpp
index a43740c..99f462c 100644
--- a/graphics/allocator/2.0/default/service.cpp
+++ b/graphics/allocator/2.0/default/service.cpp
@@ -24,5 +24,5 @@
 using android::hardware::defaultPassthroughServiceImplementation;
 
 int main() {
-    return defaultPassthroughServiceImplementation<IAllocator>();
+    return defaultPassthroughServiceImplementation<IAllocator>(4);
 }