Disallow pruning system shared static libraries

Shared libraries on the system image are assumed to always exist,
so do not prune them to free storage space.

Also fixes invalid uses of PackageSettingBase#getUserState

Bug: 128426859

Test: atest StaticSharedLibsHostTests
Test: atest EphemeralTest

Change-Id: I096749af6d7a5916ced433df60a33e2d5b8a7975