Tidy resources related to app branding.

Include in build strings for both Files and Downloads so they
get translated as part of the core app. Vendors can choose
which branding they want by simpling referencing the correct
@string/files_label or @string/downloads_label in an overlay.

Also, remove specific icon for Picker allowing it to fall back
to the app icon...visually a no-op.

Bug: 35212289
Test: Build install see "Downloads".
Change-Id: I604fef3b0ca8860db22925143350faec1996d983
(cherry picked from commit 8474059d)