Increase droiddoc -J-Xmx to 1600MB.

As we occasionally see  java.lang.OutOfMemoryError when running
doc-comment-check-docs.

Bug: 25787786
Change-Id: I6c505c96ec36d09d28d400e03027b169870c49bb
diff --git a/core/droiddoc.mk b/core/droiddoc.mk
index 74f7d8b..1e9de1c 100644
--- a/core/droiddoc.mk
+++ b/core/droiddoc.mk
@@ -171,7 +171,7 @@
 		javadoc \
                 -encoding UTF-8 \
                 \@$(PRIVATE_SRC_LIST_FILE) \
-                -J-Xmx1280m \
+                -J-Xmx1600m \
                 -XDignore.symbol.file \
                 $(PRIVATE_PROFILING_OPTIONS) \
                 -quiet \