Angler: Set boot-dex2oat-threads

The fallback on boot to the image property has been removed. Set
it explicitly now.

Change-Id: I7e095168ede2478e424292c88e29d2e9b1c59574
diff --git a/device.mk b/device.mk
index b1a0f81..6c08881 100755
--- a/device.mk
+++ b/device.mk
@@ -356,6 +356,7 @@
 
 # limit dex2oat threads to improve thermals
 PRODUCT_PROPERTY_OVERRIDES += \
+    dalvik.vm.boot-dex2oat-threads=4 \
     dalvik.vm.dex2oat-threads=2 \
     dalvik.vm.image-dex2oat-threads=4