[pm] properly nullify pkg when DELETE_KEEP_DATA

Previously the pkg is not nullified if the "--user" is not passed in. This
CL makes sure that the behavior is consistent between "uninstall -k" and
"uninstall -k --user 0" (when there is only user 0), as well as after
reboot.

Also making sure that signingInfo is in the PackageInfo query result
(for potential future usages like package-unarchive).

BUG: 288142708
Test: atest com.android.server.pm.PackageManagerTests
Test: manual and will add a cts
Change-Id: Iea14d834ab3b32e4e9f7bb100dacacd4badc17e2