Add delay to avatar update callback

[pinarol]

Continuation of #459

Description

Adding a delay to wait until the server has updated the selected avatar before updating the UI. Without the delay the cache busting remains insufficient to capture the new avatar. With less than 800 ms, we can still see the issue (see comment). Hopefully, we can remove this delay soon.

Testing Steps

Use the steps in #459

Check if the avatar in the demo app gets refreshed

[wpmobilebot]

📲 You can test the changes from this Pull Request in Gravatar UIKit Prototype Build by scanning the QR code below to install the corresponding build.

	App Name	Gravatar UIKit Prototype Build
	Build Number	1395
	Version	1.0
	Bundle ID	com.automattic.gravatar-sdk-demo-uikit.prototype-build
	Commit	fc8f657a98e7cf825dbd59bf93b378b8c6569406
	App Center Build	Gravatar SDK Demo - UIKit #213

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[wpmobilebot]

📲 You can test the changes from this Pull Request in Gravatar SwiftUI Prototype Build by scanning the QR code below to install the corresponding build.

	App Name	Gravatar SwiftUI Prototype Build
	Build Number	1395
	Version	1.0
	Bundle ID	com.automattic.gravatar-sdk-demo-swiftui.prototype-build
	Commit	fc8f657a98e7cf825dbd59bf93b378b8c6569406
	App Center Build	Gravatar SDK Demo - SwiftUI #213

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.