This is part of an initiative to use "app" instead of "applications", see: Teams/Design/initiatives#123
Redo of !1539 (closed) that was closed by a ghosting user.
Replaces "application" with "app" in user facing strings in these panels:
- applications
- camera, location, microphone
- default-apps
- multitasking
- notifications
- removable-media
- search
- usage
- user-accounts
Fixes: #2208 (closed).