Treat core_platform as stable unless module uses legacy

The sdk_version: "core_platform" refers to the stable core platform
unless the module is in the exception list. This change makes sure that
CheckStableSdkVersion() reflects that behavior.

Bug: 180399951
Test: m nothing
Change-Id: Ia0b1e13322352b87f5a3c6621e37f23ba637ffb6
diff --git a/apex/apex_test.go b/apex/apex_test.go
index 3e5ba7f..44ddbcb 100644
--- a/apex/apex_test.go
+++ b/apex/apex_test.go
@@ -1971,7 +1971,7 @@
 				java_library {
 					name: "myjar",
 					srcs: ["foo/bar/MyClass.java"],
-					sdk_version: "core_platform",
+					sdk_version: "test_current",
 					apex_available: ["myapex"],
 				}
 			`,
@@ -2018,13 +2018,16 @@
 				java_library {
 					name: "myjar",
 					srcs: ["foo/bar/MyClass.java"],
-					sdk_version: "core_platform",
+					sdk_version: "test_current",
 					apex_available: ["myapex"],
 				}
 			`,
 		},
 		{
-			name:          "Updatable apex with non-stable transitive dep",
+			name: "Updatable apex with non-stable transitive dep",
+			// This is not actually detecting that the transitive dependency is unstable, rather it is
+			// detecting that the transitive dependency is building against a wider API surface than the
+			// module that depends on it is using.
 			expectedError: "compiles against Android API, but dependency \"transitive-jar\" is compiling against private API.",
 			bp: `
 				apex {