The aapt and aidl tools are prebuilt when we're doing an apps-only build, so don't try to build them.

Change-Id: I9896b11a9b90cbbbe2170243c8bdbc40231dfbff
diff --git a/tools/aapt/Android.mk b/tools/aapt/Android.mk
index 499a7ac..b339a2c 100644
--- a/tools/aapt/Android.mk
+++ b/tools/aapt/Android.mk
@@ -4,6 +4,9 @@
 # Android Asset Packaging Tool
 #
 
+# This tool is prebuilt if we're doing an app-only build.
+ifeq ($(TARGET_BUILD_APPS),)
+
 LOCAL_PATH:= $(call my-dir)
 include $(CLEAR_VARS)
 
@@ -53,3 +56,4 @@
 
 include $(BUILD_HOST_EXECUTABLE)
 
+endif # TARGET_BUILD_APPS