Do not vibe when the default notification sound is Silent.

(This relates to the new vibration fallback behavior, where
notifications that expect to make a sound should always
vibrate in vibrate mode. We should not vibrate if the
notification's sound is silent, but we should also not
vibrate if the notification uses the default sound and the
default is silent.)

Bug: 7537077
Change-Id: I08e149c8c00ef2d2f61e418d88a086cb5e9cf241