Fixed accessibility issues in Wi-Fi SSID view

- 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 SettingsUnitTests:AddNetworkFragmentTest
  atest WifiConfigController2Test
Change-Id: I6cc6b197864511ee2c1c92affc67698b1761f11d