Hide tethering option when in a guest account

- Before this CL, users can share their mobile data through enabling
  tethering in the USB menu when in a guest account.
  It will cause main users to spend their money unexpectedly.

  This CL will hide the tethering option on the USB menu when in the
  guest account.
- Add test cases.

Bug: 166125765
Test: 1. make -j42 RunSettingsRoboTests. 2. Switch to a guest account
to verify whether the tethering option is hiding.

Change-Id: I7523b3f3c3a1372bb128a58a6a7fac973d27cfbe

tests/robotests/src/com/android/settings/connecteddevice/usb/UsbBackendTest.java

@@ -181,4 +181,24 @@ public class UsbBackendTest {
 
         assertThat(usbBackend.areFunctionsSupported(UsbManager.FUNCTION_MTP)).isTrue();
     }
+
+    @Test
+    public void areFunctionsDisallowedByNonAdminUser_isAdminUser_returnFalse() {
+        when(mUserManager.isAdminUser()).thenReturn(true);
+
+        final UsbBackend usbBackend = new UsbBackend(mContext, mUserManager);
+
+        assertThat(usbBackend.areFunctionsDisallowedByNonAdminUser(
+                UsbManager.FUNCTION_RNDIS)).isFalse();
+    }
+
+    @Test
+    public void areFunctionsDisallowedByNonAdminUser_isNotAdminUser_returnTrue() {
+        when(mUserManager.isAdminUser()).thenReturn(false);
+
+        final UsbBackend usbBackend = new UsbBackend(mContext, mUserManager);
+
+        assertThat(usbBackend.areFunctionsDisallowedByNonAdminUser(
+                UsbManager.FUNCTION_RNDIS)).isTrue();
+    }
 }