Add UserHandle.getAppId to SystemApi

This is consistent with other methods in this class, and unbundled
system apps such as the network stack need this API to verify their
callers regardless of the userid. The method is already marked as
TestApi.

Test: m
Bug: 123655057
Change-Id: I0ecef4abcd850dc641cd70681a56cb038778c04c

api/system-current.txt

@@ -4303,6 +4303,7 @@ package android.os {
   }
 
   public final class UserHandle implements android.os.Parcelable {
+    method public static int getAppId(int);
     method public int getIdentifier();
     method @Deprecated public boolean isOwner();
     method public boolean isSystem();

core/java/android/os/UserHandle.java

@@ -226,6 +226,7 @@ public final class UserHandle implements Parcelable {
      * @hide
      */
     @TestApi
+    @SystemApi
     public static @AppIdInt int getAppId(int uid) {
         return uid % PER_USER_RANGE;
     }