Do not throw new incremental not found exceptions

For incremental hardening, our primary focus is to ensure the system
does not crash. Rather than throw new exceptions when querying from
incrementally installed packages we should return null values and not
subvert API expectations by throwing exceptions in places that
normally cannot fail.

This may cause UI issues when an app loads the resources of a
different, incrementally installed package. We can introduce new APIs
to allow system apps to determine when a package not being fully
present would cause issues on a case-by-case basis.

We will be able to tell from logcat and bugreports where the failed
read occurred in the resources.arsc.

Bug: 169167078
Test: builds
Change-Id: Idfd9ad11b64fc9e8dce9d0fa7ab9b6e4f6083d41