dalvik sysprop settings are optional

dalvik sysprop settings in these mk files are changed to optional ones
using the new 'a ?= b' syntax, to better express the intent that these
provides safe default values which will be used when they are set in
a product-specific mk file.

This is also in preparation for
I9c073a21c8257987cf2378012cadaeeeb698a4fb, which when lands will
prohibit duplicated sysprop assignments.

Bug: 117892318
Bug: 158735147
Test: m
Change-Id: Ia3528c0c2f9715ff10b85b549d739c9263ab6a7e
diff --git a/build/phone-xhdpi-6144-dalvik-heap.mk b/build/phone-xhdpi-6144-dalvik-heap.mk
index 2bacc4a..f08830b 100644
--- a/build/phone-xhdpi-6144-dalvik-heap.mk
+++ b/build/phone-xhdpi-6144-dalvik-heap.mk
@@ -16,10 +16,10 @@
 
 # Provides overrides to configure the Dalvik heap for a 6GB phone
 
-PRODUCT_PROPERTY_OVERRIDES += \
-    dalvik.vm.heapstartsize=16m \
-    dalvik.vm.heapgrowthlimit=256m \
-    dalvik.vm.heapsize=512m \
-    dalvik.vm.heaptargetutilization=0.5 \
-    dalvik.vm.heapminfree=8m \
-    dalvik.vm.heapmaxfree=32m
+PRODUCT_VENDOR_PROPERTIES += \
+    dalvik.vm.heapstartsize?=16m \
+    dalvik.vm.heapgrowthlimit?=256m \
+    dalvik.vm.heapsize?=512m \
+    dalvik.vm.heaptargetutilization?=0.5 \
+    dalvik.vm.heapminfree?=8m \
+    dalvik.vm.heapmaxfree?=32m