Allow system to retrieve persistent preferred activities

This CL allows system code to retrieve persistent preferred activities
(default intent handlers set by a Device Owner or Profile Owner app).
Settings will use this information to indicate which default apps were
set by the admin.

There is no public PackageManager API. Settings will call via the
IPackageManager interface directly.

Bug: 32692748
Test: Will be CTS-verifier-tested together with Settings

Change-Id: Ibd0a39f13852a9117836ca75cc0882e4cbe0ec1d