have "make clean" also remove android's/gradle's output dirs

esp. removing the jniLib one is important, as otherwise it is easy to
package both x86 as well as arm libs into the apk, making it huuuge :-)

Change-Id: I6364b82800a449682b618f0bb98a47825dcb0742

Makefile.in

@@ -179,6 +179,14 @@ clean-build:
 ifneq ($(CROSS_COMPILING),)
 	rm -fr $(INSTDIR_FOR_BUILD)
 	rm -fr $(WORKDIR_FOR_BUILD)
+ifeq ($(OS),ANDROID)
+	rm -fr $(SRCDIR)/android/source/obj
+	rm -fr $(SRCDIR)/android/source/jniLibs
+	rm -fr $(SRCDIR)/android/source/build
+	rm -fr $(SRCDIR)/android/source/assets
+	rm -fr $(SRCDIR)/android/source/assets_fullUI
+	rm -fr $(SRCDIR)/android/source/assets_strippedUI
+endif
 endif
 
 include $(SRCDIR)/compilerplugins/Makefile.mk