Make native.bridge property overridable

When a device uses native bridge, it may be included in the system
image, so overriding by vendor/build.prop is not an option.
Example of overriding can be seen here: ag/15881132

Test: build cf_x86_64_phone-userdebug with native-bridge and launch, observe
1. ro.dalvik.vm.native.bridge=native_bridge.so moved from
vendor/build.prop to system/build.prop
2. the prop is correctly set at runtime
3. translated apps work
Bug: 197153442

Change-Id: I6cd566dd4e0fac181e309cb7f282d086de09075b
diff --git a/target/product/runtime_libart.mk b/target/product/runtime_libart.mk
index b511aa6..1d52b9f 100644
--- a/target/product/runtime_libart.mk
+++ b/target/product/runtime_libart.mk
@@ -93,7 +93,7 @@
     dalvik.vm.appimageformat=lz4
 
 PRODUCT_SYSTEM_PROPERTIES += \
-    ro.dalvik.vm.native.bridge=0
+    ro.dalvik.vm.native.bridge?=0
 
 # Different dexopt types for different package update/install times.
 # On eng builds, make "boot" reasons only extract for faster turnaround.