Move NotificationsInteractor & related repos to shared.

We need a way to access notifications-related secure settings in new
sysui recommended architecture code. WallpaperPicker already had code
for this (in the form of a NotificationsInteractor,
NotificationsRepository and SecureSettingsRepository), so instead of
having multiple sources of truth for the same type of data, I moved
these to the shared package so both SystemUI and WallpaperPicker can
access them.

I also renamed NotificationsInteractor and NotificationsRepository to
NotificationsSettingsInteractor and NotificationSettingsRepository
respectively, to better reflect the kind of data they handle.

Added the fake repository to a new CustomizationTestUtils lib.

This move also includes a very small behavior change - there was a
circular dependency between Notifications[Settings]Interactor and
NotificationsSnapshotRestorer, which deals with restoring setting when
the "Reset" button is pressed in WallpaperPicker. Since the snapshot
restorer only needs to exist in WallpaperPicker code, I split it from
the interactor and made it collect the settings flow from the interactor
directly (see other CLs in topic).
I tested that this works correctly by running WallpaperPickerGoogle and
doing the following steps:
- open Lock screen customization page
- toggle "Show notifications on the lockscreen"
- lock the device to verify that the correct setting is applied
- unlock the device and press "Reset"
- verify that the toggle state is updated
- lock the device to verify that the old setting is applied

Note: The test associated with NotificationsSettingsRepository is
currently in WallpaperPickerTests as opposed to CustomizationTests, and
it's complicated to move it so it will stay there for now. See
b/315806189.

Bug: 293167744
Test: manual (see steps above) + atest
NotificationSettingsRepositoryTest
Flag: NONE

Change-Id: I4d4bacd16ce070823d61de601b03f0d0ec63e822