Export JAVAC_WRAPPER environment.

To allow to inject a wrapper program for compiling java code, let
me allow soong to export JAVAC_WRAPPER.

Test: Set JAVAC_WRAPPER and ANDROID_COMPILE_WITH_JACK=false envs,
Test: and make -j 32

Change-Id: Ib77fe00bd781f8f4ddf965c5e7905eab466a057a
diff --git a/ui/build/make.go b/ui/build/make.go
index 89e03f7..a8d4483 100644
--- a/ui/build/make.go
+++ b/ui/build/make.go
@@ -89,6 +89,7 @@
 		// compiler wrappers set up by make
 		"CC_WRAPPER",
 		"CXX_WRAPPER",
+		"JAVAC_WRAPPER",
 
 		// ccache settings
 		"CCACHE_COMPILERCHECK",