Ensure APEX's Java deps use stable SDKs.

Test: m
Bug: 153333044
Change-Id: Ib1acf3073e96fe23c92d292ec0b1a91e2cd408db

java/sdk.go

@@ -169,9 +169,12 @@ func (s sdkSpec) stable() bool {
 		return false
 	}
 	switch s.kind {
+	case sdkNone:
+		// there is nothing to manage and version in this case; de facto stable API.
+		return true
 	case sdkCore, sdkPublic, sdkSystem, sdkModule, sdkSystemServer:
 		return true
-	case sdkNone, sdkCorePlatform, sdkTest, sdkPrivate:
+	case sdkCorePlatform, sdkTest, sdkPrivate:
 		return false
 	default:
 		panic(fmt.Errorf("unknown sdkKind=%v", s.kind))