Isolate IPackageManager implementation

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