Create two sentinel api levels

This relands aosp/2470068. The original CL was submitted as part of a
stack that broke tm and udc. Those branches still contain soong modules
with min_sdk_version of type (kind+level).

Test: m nothing on tm and udc (via go/abtd)
Bug: 208456999

Change-Id: I8e013ec10530372f70f0ab0505b7eebeee2b360b

android/config.go

@@ -52,6 +52,15 @@ var StringDefault = proptools.StringDefault
 // FutureApiLevelInt is a placeholder constant for unreleased API levels.
 const FutureApiLevelInt = 10000
 
+// PrivateApiLevel represents the api level of SdkSpecPrivate (sdk_version: "")
+// This api_level exists to differentiate user-provided "" from "current" sdk_version
+// The differentiation is necessary to enable different validation rules for these two possible values.
+var PrivateApiLevel = ApiLevel{
+	value:     "current",             // The value is current since aidl expects `current` as the default (TestAidlFlagsWithMinSdkVersion)
+	number:    FutureApiLevelInt + 1, // This is used to differentiate it from FutureApiLevel
+	isPreview: true,
+}
+
 // FutureApiLevel represents unreleased API levels.
 var FutureApiLevel = ApiLevel{
 	value:     "current",