Expose the minimum supported SDK version to make.

Right now this is hard coded in two places (here and in
core/binary.mk). Keep it in one place so it's easier to change.

Test: make checkbuild
Bug: None
Change-Id: I2a2c784d4c667b326f871e6144db92753c16c85f
diff --git a/android/makevars.go b/android/makevars.go
index 482fbde..024e015 100644
--- a/android/makevars.go
+++ b/android/makevars.go
@@ -19,11 +19,20 @@
 	"fmt"
 	"io/ioutil"
 	"os"
+	"strconv"
 
 	"github.com/google/blueprint"
 	"github.com/google/blueprint/proptools"
 )
 
+func init() {
+	RegisterMakeVarsProvider(pctx, androidMakeVarsProvider)
+}
+
+func androidMakeVarsProvider(ctx MakeVarsContext) {
+	ctx.Strict("MIN_SUPPORTED_SDK_VERSION", strconv.Itoa(ctx.Config().MinSupportedSdkVersion()))
+}
+
 ///////////////////////////////////////////////////////////////////////////////
 // Interface for other packages to use to declare make variables
 type MakeVarsContext interface {