Readd removed AOSP string

This string seems to be still used in some layouts,
it was removed when merging commits from CAF.

Build was not broken because it used the translated strings.
Add it back to be on the safe side.

Change-Id: I5bc0909d02685da9cac4fe0ea50429a0011f4182
Dialer: EL translations

* Improved strings based on Google/AOSP strings

Change-Id: I79b55fe72025c7cd88b5b39e6f7ef745b6087a68
[3/3] Base: Incoming Call in Background