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/config.go b/android/config.go
index ee2f40f..330669d 100644
--- a/android/config.go
+++ b/android/config.go
@@ -378,6 +378,10 @@
 	return strconv.Itoa(c.PlatformSdkVersionInt())
 }
 
+func (c *config) MinSupportedSdkVersion() int {
+	return 9
+}
+
 // Codenames that are active in the current lunch target.
 func (c *config) PlatformVersionActiveCodenames() []string {
 	return c.ProductVariables.Platform_version_active_codenames