Display: add System Settings Flow & Delegate

Added System Settings Flow that allows to observe and react on
Integer System settings values using Kotlin Coroutines. Also add
System Settings Integer delegate.

Flag: EXEMPT only usage guarded by another flag
Test: atest com.android.settingslib.spaprivileged.settingsprovider
Test: atest --iteration 30 -c frameworks/base/packages/SettingsLib/SpaPrivileged/tests/robotests
Test: verified that SettingsSystemIntegerFlow works with
`display_color_mode` Int setting w/ and /wo set value.
Bug: 390644464

Change-Id: Iacda050b9ff44bda4358756848b15319c699eef1