Notification Power Widget: Fix silent mode detection

Recent upstream changes have broken detection of silent mode, so
when for example using the device vol keys to set silent mode the
wrong icon is displayed.
This is caused by a faulty assumption about vibration modes. When
ringer mode is set to silent, vibration mode is ignored by Android.
Thus, when comparing Ringers we can not compare vibration modes when
ringer mode is silent. This has been true since API level 1 but since
Google never can seem to decide how to handle silent/vibrate/ring
consistently it's understandable that bugs like this happen.

Change-Id: Ie3daddd51a7bba0dea86f3f58f6963d22e34cc6d