Fix notifications undo system (1/2)

Adjust so NotficationSettingsRepository in frameworks/base emits the
show notifications boolean in a suspend function, after it is retrieved
from SecureSettingsRepository, instead of returning a default value
immediately.

Also added missing test NotificationsSnapshotRestorerTest.

Flag: EXEMPT bug fix
Bug: 326440143
Test: manually verified notifications toggle sets and resets correctly
Test: manually verified reset button shows and hides correctly
Test: existing notifications unit tests pass
Test: new ThemePicker test NotificationsSnapshotRestorerTest
Change-Id: If580a54fdb920a6a17f7c605caf5359a868f0ec0