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