More performance improvements

Instead of using `x := $(x) foo`, make sure `x` is initialized first,
then use `x += foo`.

For a aosp-master/aosp_crosshatch-userdebug build, this takes the build
from 49s to 33s (no significant difference in peak memory)

Bug: 158488548
Test: build-aosp_crosshatch.ninja is the same before/after
Change-Id: I41fb7611030dd3ba824f896a9c9eadbdd218f464
diff --git a/core/soong_java_prebuilt.mk b/core/soong_java_prebuilt.mk
index b15e7f6..e4c84e0 100644
--- a/core/soong_java_prebuilt.mk
+++ b/core/soong_java_prebuilt.mk
@@ -173,4 +173,4 @@
 		$(hide) echo $(PRIVATE_EXPORTED_SDK_LIBS) | tr ' ' '\n' > $@,\
 		$(hide) touch $@)
 
-SOONG_ALREADY_CONV := $(SOONG_ALREADY_CONV) $(LOCAL_MODULE)
+SOONG_ALREADY_CONV += $(LOCAL_MODULE)