[Role Logic Move] Lift user from RoleBehavior::getDefaultHolders

This change modifies the following methods:

- RoleBehavior::getDefaultHolders[AsUser]
- Role::getDefaultHolders[AsUser]
- RoleBehavior::getFallbackHolder[AsUser]
- Role::getFallbackHolder[AsUser]

...by making them multi-user aware. I.e., lift any references to
Process.myUserHandle() out of them into method arguments.

Bug: 302563690
Test: atest CtsRoleTestCases
Change-Id: I751423aa19b5e5733aa10793ca722d4300e1f0ab