inputmethod: remove the workarounds

- Do not set emit the enabledChanged of the text-input-v2 when it
  requests us to show input panel, it is already enabled when you get
  this request
- If we get the request to show input panel, reset the inputContext,
  there's two reason we can get this event: 1) when keyboard focus is
  activated or 2) when keyboard focus is changed to text field in same
  surface. In both cases, information in input method context is not
  valid anymore and should be reset.
- Also instead of ignoring the update_state events from client, adopt
  input method accordingly.