Restore lazy loading of LoadedApk resources

This was broken in 8fff544ec349593cfa61ae2f5ec59419fff666b6. Since that change,
resources are always initialized in getPackageInfo(), even if they haven't been
loaded before (mResources == null).

It shouldn't be necessary in *any* case to preload the resources at this point.
Anything using the resources should call LoadedApk.getResources(), which will
initialize them just when needed. Setting mResources to null resets the cache
and will force a reload the next time the resources are requested.

Change-Id: I9bc9986297478e99752b1e2b9bd04dab3dcc9656