Fixed accessibility issues in Wi-Fi SSID view for SUW

- Keep the Save button enabled at all times

- Show "Enter the SSID" to remind the user

Bug: 386897596
Flag: EXEMPT bugfix
Test: Manual testing
  atest WifiDialogActivityTest \
        WifiConfigControllerTest
Change-Id: I577e78c34cbaa0640479adf09a916a526500fe68

tests/robotests/src/com/android/settings/wifi/WifiConfigControllerTest.java

@@ -145,14 +145,6 @@ public class WifiConfigControllerTest {
                 .isEqualTo(View.GONE);
     }
 
-    @Test
-    public void isSubmittable_noSSID_shouldReturnFalse() {
-        final TextView ssid = mView.findViewById(R.id.ssid);
-        assertThat(ssid).isNotNull();
-        ssid.setText("");
-        assertThat(mController.isSubmittable()).isFalse();
-    }
-
     @Test
     public void isSubmittable_longPsk_shouldReturnFalse() {
         final TextView password = mView.findViewById(R.id.password);