Revert "Revert "Update signature permissions when package signing is changed.""

This reverts commit 2eaf8488.
Reason for revert: Fixed SigningDetails.equals() NPE in b/290558495

Fixes: 288515966
Test: KeySetHostTest#testUpgradeDefinerSigPerm{Gained,Lost}
Test: Reproduction steps in b/290558495
Change-Id: I694a565957576633ba7e1424c49a77d97fcdff26