To avoid leaking IPackageManager methods directly into
PackageManagerService, moves the IPM implementation into an inner
class similar to PackageManagerInternal.
This will help enforce snapshot consistency by ensuring that all
non-IPM PMS methods can always take in a Computer instance to re-use.
Bug: 217961172
Test: presubmit
Change-Id: Ibf7bd3d1d90a40786279763ba8d62eb9348bb760