Use build_number from file

Improve incremental ninja builds by keeping the command line the same
across builds.

Change-Id: If9dfcee74a4d282c183f9669e14a89104c115a71
diff --git a/tools/aapt/Android.mk b/tools/aapt/Android.mk
index d551c8e..0b456e6 100644
--- a/tools/aapt/Android.mk
+++ b/tools/aapt/Android.mk
@@ -69,7 +69,7 @@
     libziparchive-host \
     libbase
 
-aaptCFlags := -DAAPT_VERSION=\"$(BUILD_NUMBER)\"
+aaptCFlags := -DAAPT_VERSION=\"$(BUILD_NUMBER_FROM_FILE)\"
 aaptCFlags += -Wall -Werror
 
 ifeq ($(HOST_OS),linux)