Check for `focus` and `blur` methods before calling them in answer-area-renderer

Summary: If your answer-area was a radio, which doesn't have a `blur` method, itemRenderer would
sometimes throw an exception and, later, crash the mobile app. Now, we check for the existence
of these methods before calling them.

Test Plan:
- Run mobile app.
- Go to "Groups of tens".
    - Arrow around the inputs.
- Enable <Table> inputs (apply patch).
    - Create some inputs.
    - Arrow around them.
    - Verify that app doesn't crash.

Auditors: jack

Subscribers: kasra