Use `.gravatarBlue` for selected avatar's border

[pinarol]

Closes #

Description

Something I noticed during the Jetpack integration is that we use .accentColor for the selection border, and that can vary from app to app:

We don't want that, we want to use .gravatarBlue according to designs.

Testing Steps

Just check the border in the demo app and verify it's .gravatarBlue, it's slightly different than the default accent color.

[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	1493
	Version	1.0
	Bundle ID	com.automattic.gravatar-sdk-demo-uikit.prototype-build
	Commit	c4bff4fc6007448fb57d6370aed503edb446f083
	App Center Build	Gravatar SDK Demo - UIKit #268

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	1493
	Version	1.0
	Bundle ID	com.automattic.gravatar-sdk-demo-swiftui.prototype-build
	Commit	c4bff4fc6007448fb57d6370aed503edb446f083
	App Center Build	Gravatar SDK Demo - SwiftUI #268

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