Prohibit dependencies outside of uses_sdks

When an APEX is built with uses_sdks, any depedndency from the APEX to
the outside of the APEX should be from the SDKs that the APEX is built
against.

Bug: 138182343
Test: m

Change-Id: I1c2ffe8d28ccf648d928ea59652c2d0070bf10eb

android/sdk.go

@@ -44,6 +44,17 @@ func (s SdkRef) Unversioned() bool {
 	return s.Version == ""
 }
 
+// String returns string representation of this SdkRef for debugging purpose
+func (s SdkRef) String() string {
+	if s.Name == "" {
+		return "(No Sdk)"
+	}
+	if s.Unversioned() {
+		return s.Name
+	}
+	return s.Name + string(SdkVersionSeparator) + s.Version
+}
+
 // SdkVersionSeparator is a character used to separate an sdk name and its version
 const SdkVersionSeparator = '@'