Catch potential NPE in RingtoneFactory.

This has been seen intermittently where RingtoneManager is throwing an
NPE while calling getRingtone.
Adding catch blocks to ensure in the worse case we don't crash telecom.
Bug: 178151542
Test: Compile; this is a speculative fix to an intermittent problem.

Change-Id: I88cb4314d65f7b4bfd383bf0e38dda6ff53a7a72