Add null check for package settings

A few lines down "ps" is run through a null check, so apparently there
could be some screwy system conditions where this is encountered. And,
in fact, this was encountered through an updated system pacakge that
managed to delete the package from /system/app during testing. It left
the PackageSetting in mDisabledSysPackages which would cause a deference
of a null variable.

Change-Id: Ic7faca569abd16a87a8a69c20df08c11a274a001