Enforce hidden api usage in vendor (soong)

From aosp/588578, if vendor app doesn't fill sdk_version, build scirpt overwrites system_current to sdk_version.
But there is no similar enforcement in soong.

So, as make does, sdkVersion() returns "system_current" if it is device-specific or soc-specific module. and sdk_version is not filled.

Test: pass soong test
Bug: 132780927

Change-Id: I69bb3d7bfcf7c27c2db4d0efbe046f9c7879b4bc

java/aar.go

@@ -499,7 +499,7 @@ type AARImport struct {
 }
 
 func (a *AARImport) sdkVersion() string {
-	return String(a.properties.Sdk_version)
+	return proptools.StringDefault(a.properties.Sdk_version, defaultSdkVersion(a))
 }
 
 func (a *AARImport) minSdkVersion() string {