Fix temporary directory path in Android.

We should use /data/misc/update_engine/tmp instead of /data/local/tmp.

Bug: 24478450
TEST=Delta update nolonger permission denied.

Change-Id: I9a8096e602320195d30147971def5145bd82d775
diff --git a/common/platform_constants.h b/common/platform_constants.h
index 8331064..d1786ff 100644
--- a/common/platform_constants.h
+++ b/common/platform_constants.h
@@ -47,6 +47,9 @@
 // response. Empty if not supported.
 extern const char kOmahaResponseDeadlineFile[];
 
+// The stateful directory used by update_engine.
+extern const char kNonVolatileDirectory[];
+
 }  // namespace constants
 }  // namespace chromeos_update_engine