Change app ID type in BackgroundContentsService from string16 to string

Remove a lot of utf conversions.

Bug: none
Change-Id: I6275609fd29da15aa10d6c1f38bb8fc9af4e0239
Reviewed-on: https://chromium-review.googlesource.com/780375


Commit-Queue: Evan Stade <estade@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518286}