Add an order-only dependency on the build number file

Remote execution and other tools can be confused by references to
build_number.txt without a dependency.  Add an order-only dependency,
which maintains the current behavior.

Test: BUILD_NUMBER=1 && m aapt && aapt version # shows 1
      BUILD_NUMBER=2 && m aapt && aapt version # shows 1
      rm out/soong/.intermediates/frameworks/base/tools/aapt/aapt/linux_glibc_x86_64/aapt
      BUILD_NUMBER=2 && m aapt && aapt version # shows 2
Change-Id: Icfa98d6840b1dc2e273ba29c33011635d1cf93b1
diff --git a/android/variable.go b/android/variable.go
index 9cbe624..25c94bc 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -140,9 +140,8 @@
 	// Suffix to add to generated Makefiles
 	Make_suffix *string `json:",omitempty"`
 
-	BuildId             *string `json:",omitempty"`
-	BuildNumberFromFile *string `json:",omitempty"`
-	DateFromFile        *string `json:",omitempty"`
+	BuildId         *string `json:",omitempty"`
+	BuildNumberFile *string `json:",omitempty"`
 
 	Platform_version_name                     *string  `json:",omitempty"`
 	Platform_sdk_version                      *int     `json:",omitempty"`
@@ -345,7 +344,7 @@
 
 func (v *productVariables) SetDefaultConfig() {
 	*v = productVariables{
-		BuildNumberFromFile: stringPtr("123456789"),
+		BuildNumberFile: stringPtr("build_number.txt"),
 
 		Platform_version_name:             stringPtr("Q"),
 		Platform_sdk_version:              intPtr(28),