Fix crash by "adb shell pm list permissions -f|-s"

pm command makes the assumption that every permission should
contain android:description and android:label attributes while
they are not mandatory. If a permission does not contain these
two attributes, we get "android.content.res.
Resources$NotFoundException: String resource ID #0x0"
followed by a NPE when using -f or -s options.

With the following change, users will get "null" in output for
respective fields.

Change-Id: I4e7f407592fa071abdab1d979775f46ec27dc9d2