Expose SdkExtension in the system SDK

Also tweak the javadoc a little bit.

Bug: 137191822
Bug: 143937447
Test: treehugger
Change-Id: Iae1b025685344c2ffe4ed1089850b447c82a6b83
Merged-In: Iae1b025685344c2ffe4ed1089850b447c82a6b83

Android.bp

@@ -563,6 +563,7 @@ filegroup {
         "core/java/android/annotation/Nullable.java",
         "core/java/android/annotation/IntDef.java",
         "core/java/android/annotation/IntRange.java",
+        "core/java/android/annotation/SystemApi.java",
         "core/java/android/annotation/UnsupportedAppUsage.java",
         "core/java/com/android/internal/annotations/GuardedBy.java",
         "core/java/com/android/internal/annotations/VisibleForTesting.java",

apex/sdkext/framework/java/android/os/ext/SdkExtensions.java

@@ -17,13 +17,23 @@
 package android.os.ext;
 
 import android.annotation.IntDef;
+import android.annotation.SystemApi;
 import android.os.Build.VERSION_CODES;
 import android.os.SystemProperties;
 
 import java.lang.annotation.Retention;
 import java.lang.annotation.RetentionPolicy;
 
-/** @hide */
+/**
+ * Methods for interacting with the extension SDK.
+ *
+ * This class provides information about the extension SDK version present
+ * on this device. Use the {@link #getExtensionVersion(int) getExtension} to
+ * query for the extension version for the given SDK version.
+
+ * @hide
+ */
+@SystemApi
 public class SdkExtensions {
 
     private static final int R_EXTENSION_INT;
@@ -31,11 +41,16 @@ public class SdkExtensions {
         R_EXTENSION_INT = SystemProperties.getInt("persist.com.android.sdkext.sdk_info", 0);
     }
 
-    /** Values suitable as parameters for {@link #getExtensionVersion(int)}. */
+    /**
+     * Values suitable as parameters for {@link #getExtensionVersion(int)}.
+     * @hide
+     */
     @IntDef(value = { VERSION_CODES.R })
     @Retention(RetentionPolicy.SOURCE)
     public @interface SdkVersion {}
 
+    private SdkExtensions() { }
+
     /**
      * Return the version of the extension to the given SDK.
      *

api/system-current.txt

@@ -6019,6 +6019,14 @@ package android.os {
 }
+package android.os.ext {
+
+  public class SdkExtensions {
+    method public static int getExtensionVersion(int);
+  }
+
+}
+
 package android.os.image {
   public class DynamicSystemClient {