Create a SetBuildFromTextStub method

This will be useful for writing unit tests

Test: go build ./android
Change-Id: If27a050c0cfd0492e0cd3a422ac33f40b7a0a6d5
diff --git a/android/config.go b/android/config.go
index 33deba5..038cd0f 100644
--- a/android/config.go
+++ b/android/config.go
@@ -1894,3 +1894,7 @@
 func (c *config) BuildFromTextStub() bool {
 	return c.buildFromTextStub
 }
+
+func (c *config) SetBuildFromTextStub(b bool) {
+	c.buildFromTextStub = b
+}