DeskClock: Protected against possible NPE from null uri.

  From BUGDUMP-648323 (12-20150130-20150206)

  "java.lang.NullPointerException: Attempt to invoke virtual method 'boolean android.net.Uri.equals(java.lang.Object)' on a null object reference
    at com.android.deskclock.Utils.isRingToneUriValid(Utils.java:685)
    at com.android.deskclock.AlarmClockFragment$AlarmItemAdapter.bindView(AlarmClockFragment.java:1014)
    at com.android.deskclock.AlarmClockFragment$AlarmItemAdapter.getView(AlarmClockFragment.java:936)
    at android.widget.AbsListView.obtainView(AbsListView.java:2346)
    ...

Change-Id: I092d4da0b29e400241eff82935b1f47080bc8d8a
(cherry picked from commit 6a34451b)