[PackageUsage] fix NPE

This is called by `mPm.getPackageUsage().maybeWriteAsync(mPm.mSettings.getPackagesLocked())` under `mPm.mLock` so it's unclear why any pkgSetting would be null. Perhaps there are other usages not shown in the stack  trace. Adding null pointer checks to avoid crashes.

BUG: 229823551
Test: builds
Change-Id: Ia33bb59824e774b02249a1e9872bc6e418dec658