Add droiddoc flag LOCAL_DROIDDOC_STUB_OUT_DIR

When calling droiddoc to generate stubs, if you use
LOCAL_DROIDDOC_STUB_OUT_DIR (instead of "-stubs" to
LOCAL_DROIDDOC_LOCAL_DROIDDOC_OPTIONS), the build system gets the chance
to clean up the stub out directory before calling doclava.

Bug: 22534021
Change-Id: Icb0a73eb51bfb1c1ecd88217ebcd39c27df1de29
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index 2786991..10be6d2 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -82,6 +82,7 @@
 LOCAL_DROIDDOC_CUSTOM_ASSET_DIR:=
 LOCAL_DROIDDOC_OPTIONS:=
 LOCAL_DROIDDOC_HTML_DIR:=
+LOCAL_DROIDDOC_STUB_OUT_DIR:=
 LOCAL_ADDITIONAL_HTML_DIR:=
 LOCAL_ASSET_DIR:=
 LOCAL_RESOURCE_DIR:=