Add isSystemUser() in UserSwitcherController.

This is a small util function to check whether the current user is the
system user.

Bug: 213906356
Test: atest UserSwitcherControllerTest
Change-Id: I53d452ea9d557d76632c74986d34ff4c4ed34c05

packages/SystemUI/src/com/android/systemui/statusbar/policy/UserSwitcherController.java

@@ -455,6 +455,13 @@ public class UserSwitcherController implements Dumpable {
         }
     }
 
+    /**
+     * Returns whether the current user is a system user.
+     */
+    public boolean isSystemUser() {
+        return mUserTracker.getUserId() == UserHandle.USER_SYSTEM;
+    }
+
     public void removeUserId(int userId) {
         if (userId == UserHandle.USER_SYSTEM) {
             Log.w(TAG, "User " + userId + " could not removed.");

packages/SystemUI/tests/src/com/android/systemui/statusbar/policy/UserSwitcherControllerTest.kt

@@ -400,4 +400,16 @@ class UserSwitcherControllerTest : SysuiTestCase() {
 
         assertEquals(fgUserName, userSwitcherController.currentUserName)
     }
+
+    @Test
+    fun isSystemUser_currentUserIsSystemUser_shouldReturnTrue() {
+        `when`(userTracker.userId).thenReturn(UserHandle.USER_SYSTEM)
+        assertEquals(true, userSwitcherController.isSystemUser)
+    }
+
+    @Test
+    fun isSystemUser_currentUserIsNotSystemUser_shouldReturnFalse() {
+        `when`(userTracker.userId).thenReturn(1)
+        assertEquals(false, userSwitcherController.isSystemUser)
+    }
 }