Settings: Use proper 64x64 XHDPI icon for Security Settings

Seems AOSP had a copy/paste derp.  The XHDPI and the HDPI icons are
the same (48x48) where the XHDPI one should be larger.  I am using the
CM10 icon now.

Change-Id: I1ad2f30400d24187848fbef0ae907fd8f71edde4