Quick affordance repository.

This CL introduces KeyguardQuickAffordanceRepository which combines
the known built-in configs and the current selections from KeyguardQuickAffordanceSelectionManager to produce the current set of
available affordances and the current selected affordances for each
slot.

KeyguardQuickAffordanceSelectionManager is just an in-memory cache at
the moment as we will flesh it out in upcoming CLs such that it actually
uses a persistence layer.

Note that the repository is meant for use for both displaying the
affordances on the lock screen and for serving content provider queries
coming from the wallpaper picker, which is why it's got both a flow and
a "get" function as opposed to a single StateFlow.

Bug: 254858696,254853190
Test: unit tests included. Tested with followup CL to make sure that
quick affordances are still showing up on the lock screen.

Change-Id: I6947e1f729c2aa6470ba21582d664ca5b3e3af70