Keyguard: Show locale string for "Unknown musician"

When the artist name or title or ablum  is "Unknown",
it cannot show locale string on the shortcut fragment
of Music on lock screen.

Add locale string defined in resource file and set it on
views.

Change-Id: Iefccc7f4462f3a850e68932a1b79971213b7d8a4
CRs-Fixed: 670040