Do not use private APIs in unfold module

Removes usages of the private/hidden APIs
in unfold module so it could be easily
reused in 1p/3p apps.

Bug: 258828766
Test: manual fold/unfold with a full screen
 app and in split screen
Change-Id: I96b3ca38e11521cc4880de74ce143cea3aa015be
diff --git a/packages/SystemUI/unfold/Android.bp b/packages/SystemUI/unfold/Android.bp
index 180b611..2e0a946 100644
--- a/packages/SystemUI/unfold/Android.bp
+++ b/packages/SystemUI/unfold/Android.bp
@@ -35,6 +35,7 @@
     ],
     kotlincflags: ["-Xjvm-default=enable"],
     java_version: "1.8",
+    sdk_version: "current",
     min_sdk_version: "current",
     plugins: ["dagger2-compiler"],
 }