Fix "pm list permissions" crash if resource string missing

The "pm list permissions" command lists detailed information about
each permission on the system, including its label and description,
both of which can be stored as translatable resource strings in APK
files.  However, it is possible that the resource identifiers for
these strings point to non-existent resources.  When this happens, the
loadText() method throws Resources.NotFoundException, causing the "pm"
command to abort prematurely, simply printing "Killed" to stdout and
a stack trace to logcat.

This commit fixes the crash by explicitly catching the
Resources.NotFoundException exception in loadText() and returning null
if it is thrown.  The loadText() method already has the potential to
return null so none of its callers need be modified.  This fixes the
crash and simply shows "label:null" and/or "description:null" in the
output if the string resource is missing.

Change-Id: I92273399e1dac6029163750d004940ee1da67428