Run doc-comment-check only for checkbuild.

Currently doc-comment-check is the single longest-running process during
the make process. It usually takes 300-400s to finish on my Z600.
What's worse, it's usually the last straggler build job.
Not running this by default can save big build time.

Bug: 7253452
Change-Id: Idc868197b59e42c6b583c66f13a0e6a1bc8d5d4e
diff --git a/Android.mk b/Android.mk
index c3126848..c98373f 100644
--- a/Android.mk
+++ b/Android.mk
@@ -606,7 +606,9 @@
 # $(gen), i.e. framework.aidl, is also needed while building against the current stub.
 $(full_target): $(framework_built) $(gen)
 
-droidcore: doc-comment-check-docs
+# Run this for checkbuild
+.PHONY: checkbuild
+checkbuild: doc-comment-check-docs
 
 # ====  static html in the pdk ==================================
 include $(CLEAR_VARS)