openharmony.inc: Do not change java preferred providers

By using weak assignment for the java preferred providers, we avoid setting
java preferred provider variables if they are already set.
We should be fine with any valid setting.

Signed-off-by: Esben Haabendal <esben.haabendal@huawei.com>

conf/distro/include/java.inc

@@ -4,5 +4,5 @@
 
 # Configuration needed by meta-java layer
 
-PREFERRED_PROVIDER_virtual/java-initial-native = "cacao-initial-native"
-PREFERRED_PROVIDER_virtual/java-native = "cacao-native"
+PREFERRED_PROVIDER_virtual/java-initial-native ?= "cacao-initial-native"
+PREFERRED_PROVIDER_virtual/java-native ?= "cacao-native"