Create new system API in UserHandle to get uid

For the incoming connectivity mainline, it need to compute the
intersection of a set of uids and appIds for the permission
control. As @UserIdInt values are internal implementation details
of the OS, any mainline modules should use strongly-typed
UserHandle arguments. Thus, create a new API that accepts
UserHandle and appId to get the computed uid.

Bug: 170598012
Test: atest FrameworksNetTests
Change-Id: I046546d2c5c76eac7a251e63e4234776975bf2bc

core/java/android/os/UserHandle.java

@@ -294,6 +294,19 @@ public final class UserHandle implements Parcelable {
         }
     }
 
+    /**
+     * Returns the uid that is composed from the userHandle and the appId.
+     *
+     * @param userHandle the UserHandle to compose the uid
+     * @param appId the AppId to compose the uid
+     * @return the uid that is composed from the userHandle and the appId
+     * @hide
+     */
+    @SystemApi
+    public static int getUid(@NonNull UserHandle userHandle, @AppIdInt int appId) {
+        return getUid(userHandle.getIdentifier(), appId);
+    }
+
     /**
      * Returns the app id (or base uid) for a given uid, stripping out the user id from it.
      * @hide