hidl-gen: include all-makefiles-under in Android.mk

Auto-generated Android.mk files for HIDL packages now include this line
at the end:

	include $(call all-makefiles-under,$(LOCAL_PATH))

b/31524912
Test: pass

Change-Id: Ic5cc94e8df2e7fd88bff18ecbe5b8b56cb0973d0
Signed-off-by: Iliyan Malchev <malchev@google.com>
diff --git a/main.cpp b/main.cpp
index 687440c..319cf63 100644
--- a/main.cpp
+++ b/main.cpp
@@ -391,6 +391,9 @@
         out << "\ninclude $(BUILD_JAVA_LIBRARY)\n";
     }
 
+    out << "\n\n"
+        << "include $(call all-makefiles-under,$(LOCAL_PATH))\n";
+
     return OK;
 }