Fixed Android.common.mk #define setting for small art

We were seeing things compile when they shouldn't have. This fixes it.  Plus added
a bit more documentation to options, set default values for the two small art related
thresholds to zero.

Change-Id: Id5db11962f3e30d8d4ee6b85d4cf7d6e66323adb
diff --git a/build/Android.common.mk b/build/Android.common.mk
index 2453fa9..7ba2e4f 100644
--- a/build/Android.common.mk
+++ b/build/Android.common.mk
@@ -58,8 +58,8 @@
 	-Wstrict-aliasing=3 \
 	-fstrict-aliasing
 
-ifeq ($(ART_LIGHT_MODE),true)
-  art_cflags += -DART_LIGHT_MODE=1
+ifeq ($(ART_SMALL_MODE),true)
+  art_cflags += -DART_SMALL_MODE=1
 endif
 
 # TODO: enable -std=gnu++0x for auto support when on Ubuntu 12.04 LTS (Precise Pangolin)