Introduce CommunalWidgetRepositoryRemoteImpl

This change adds an implementation of `CommunalWidgetRepository` for the
headless system user. It routes requests and gets widget updates via IPC
from the foreground user.

Bug: 357621815
Test: atest GlanceableHubWidgetManagerServiceTest
Test: tested on device
Flag: com.android.systemui.secondary_user_widget_host

Change-Id: Ica63406e58b69bf3d4fba783ca700cca7d4e48f1