DeskClock: Stop depending on platform APIs

* We can build with sdk_version = current after latest changes

Change-Id: Ie424ffd156e9595082dc8ad935fb1c9e8e22bd87

Android.bp

 android_app {
     name: "DeskClock",
     resource_dirs: ["res"],
+    sdk_version: "current",
     overrides: ["AlarmClock"],
     srcs: [
         "src/**/*.java",
         "gen/**/*.java",
     ],
-    platform_apis: true,
     product_specific: true,
     static_libs: [
         "androidx.annotation_annotation",