Android small icon support for unified notifications

[aurimasc]

https://jira.unity3d.com/browse/PLAT-21686
On Android notifications are required to have small icon. Unity APIs do not require it, we use app icon if not provided.
Unified notifications API does not allow to set icon as there is no such thing on iOS. However, there were user complains in the past that using something else than app icon is desirable. This PR adds ability to set small icon to NotificationCenter. This we we keep things the way they are - small icon cannot be set on notification, but user can configure the icon to be used on all notifications sent via unified APIs. If user wants to set icon per-notification, android specific notification API should be used instead of unified one.
Couple unrelated changes: one link in documentation does not work due to class not being fully qualified and a possible null exception was noticed for iOS (very low probability, only would happen in a scenario where notification would not work anyway due to not being setup properly).

Tested using Main test project (Unified scene). Icon is set up there and shows correctly. The scenario without icon is covered by automated tests.

[u-pr]

💡 Harness Review

The change is small and focused, but the Android scheduling path now overrides notification state that existing unified callers may rely on preserving.

_{Reviewed commit 92d3bd6a}

_{🤖 Helpful? 👍/👎}