Does safe authority comparison in AppsFilter

This change makes the authority comparison when adding packages to the
AppsFilter handle null authorities on a ProviderInfo. This is necessary
due to the current handling of duplicate authorities in system apps.
Instead of failing the build or removing the provider, we just nullify
the authority. This will be addressed more directly in b/143901347

Bug: 143901347
Fixes: 141867657
Test: atest AppEnumerationTests (on walleye)
Change-Id: Iabbc128cebf389b06a6f6f84536ee87492673dd9