Removing contents hidden string for public notifications

We're now only showing the notification header instead of the
repetitive contents hidden string. On Aod, we're still showing
a string however, since without it, it looked too empty.

Test: manual, set sensitive setting on lockscreen, observe notifications
Change-Id: If2bb66b9b3b366715aa68087e5c35cf1cdff3693
Fixes: 63741232