Use empty string for core image variant

Use the empty string for the core image variant so that modules
added to imageMutator do not change their build directory.

Bug: 142286466
Test: m checkbuild
Change-Id: Ida4534d9a4d6176236aaa480fed359ce27acfaa1
Merged-In: Ida4534d9a4d6176236aaa480fed359ce27acfaa1
(cherry picked from commit 72d685ee7f45e5393be44ae4159edf083ac918de)
diff --git a/android/image.go b/android/image.go
index 8424cf8..5291ce3 100644
--- a/android/image.go
+++ b/android/image.go
@@ -42,7 +42,7 @@
 	// CoreVariation is the variant used for framework-private libraries, or
 	// SDK libraries. (which framework-private libraries can use), which
 	// will be installed to the system image.
-	CoreVariation string = "core"
+	CoreVariation string = ""
 
 	// RecoveryVariation means a module to be installed to recovery image.
 	RecoveryVariation string = "recovery"