DisplayContentRepository

This is a replacement for the existing `ScreenshotPolicy` class:

    fun findPrimaryContent(displayId: Int): DisplayContentInfo

The existing code was a little too narrow focused on a single
policy, and returns only the top facing task/activity information.

For flexibility, all root tasks are provided in DisplayContentModel
as well as some support code for filtering and testing.

This also incorporates SystemUiState which provides information
fetched over binder from the main SystemUI process via
SystemUiProxy

Test: TODO
Bug: 327613051
Flag: N/A; not used yet
Change-Id: Ideee0d1891bfb28404127ba6428162a98eeb9b99