Droiddoc Support in Soong

Support Droiddoc to Soong based on core/droiddoc.mk. The non-std doclet
based droiddoc compilation output is a "real" stubs.jar instead of a
directory of java files and a timestamp file.

The std doclet based javadoc compilation output is a "empty" stubs.jar
instead of a timestamp file.

The stubs.jar will be exported to
out/target/common/obj/JAVA_LIBRARIES/$(LOCAL_MODULE)_intermediates/classes.jar
and out/target/common/docs/$(LOCAL_MODULE)-stubs.jar

A $(LOCAL_MODULE).zip file will be generated also, and is exported to
out/target/common/docs/$(LOCAL_MODULE)-docs.zip if property: installable is not set
to false.

Bug: b/70351683
Test: unittest + convert libcore docs Android.mk to Soong manually.

Change-Id: I1cffddd138a5d9d445f86a3d4a3fd4de88a2bc0f
(cherry picked from commit 78188ec622cb1ee24171455867fc58ffab91562e)
diff --git a/java/config/config.go b/java/config/config.go
index b5e574e..c5076eb 100644
--- a/java/config/config.go
+++ b/java/config/config.go
@@ -126,6 +126,8 @@
 
 	pctx.HostJavaToolVariable("JarjarCmd", "jarjar.jar")
 	pctx.HostJavaToolVariable("DesugarJar", "desugar.jar")
+	pctx.HostJavaToolVariable("JsilverJar", "jsilver.jar")
+	pctx.HostJavaToolVariable("DoclavaJar", "doclava.jar")
 
 	pctx.HostBinToolVariable("SoongJavacWrapper", "soong_javac_wrapper")