Do not use hidden API UserManager#getPrimaryUser

Use UserHandle#isSystem instead. They're not the same when
ro.fw.system_user_split is set, but for VT functionality
this change is benign.

Bug: 137202333
Test: make
Change-Id: I1a59f78810d073d1782d8ad8498a36ea85428c09
diff --git a/src/com/android/services/telephony/TelecomAccountRegistry.java b/src/com/android/services/telephony/TelecomAccountRegistry.java
index 0cb687f..e2eb35b 100644
--- a/src/com/android/services/telephony/TelecomAccountRegistry.java
+++ b/src/com/android/services/telephony/TelecomAccountRegistry.java
@@ -35,7 +35,6 @@
 import android.os.Looper;
 import android.os.PersistableBundle;
 import android.os.UserHandle;
-import android.os.UserManager;
 import android.provider.Settings;
 import android.telecom.PhoneAccount;
 import android.telecom.PhoneAccountHandle;
@@ -804,10 +803,7 @@
 
                 int userHandleId = intent.getIntExtra(Intent.EXTRA_USER_HANDLE, 0);
                 UserHandle currentUserHandle = UserHandle.of(userHandleId);
-                UserManager userManager =
-                        (UserManager) context.getSystemService(Context.USER_SERVICE);
-                mIsPrimaryUser = userManager.getPrimaryUser().getUserHandle()
-                        .equals(currentUserHandle);
+                mIsPrimaryUser = currentUserHandle.isSystem();
 
                 // Any time the user changes, re-register the accounts.
                 tearDownAccounts();