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/config.go b/android/config.go
index 32e32ae..b2bda2a 100644
--- a/android/config.go
+++ b/android/config.go
@@ -570,8 +570,8 @@
 	return String(c.productVariables.BuildId)
 }
 
-func (c *config) BuildNumberFromFile() string {
-	return String(c.productVariables.BuildNumberFromFile)
+func (c *config) BuildNumberFile(ctx PathContext) Path {
+	return PathForOutput(ctx, String(c.productVariables.BuildNumberFile))
 }
 
 // DeviceName returns the name of the current device target