Update german translation

Adds new translation strings for the Ring lockscreen style (which I have
called "Blasen" - bubbles - in the translation).

Also fixes some other strings as followup to
cf5e8827 (see discussion for Change
I48a73c92b6f6bbd1cc5dc5ab8aac30615a636adc). This means:
- Using "Bildschirm" instead of "Display"
- Using "Auslöseton" instead of "Verschlusston"
- Using "Symbol" instead of "Icon"
- Made lockscreen_widgets_note_summary clearer
- Fixed a typo (gestures_error_loading =~ s/bei/beim/)

Change-Id: Id767ed0eed3ec36e608e103a25f3243e8c441cfb