fix: make header subviews subscribe to transition

[WoLewicki]

Description

Added change to ScreenStackHeaderConfig.java to expose the Toolbar, of which the ScreenStackHeaderSubviews are the children. It is needed since the Toolbar is not a child of Screen in the view hierarchy, therefore its children are not subscribed to the transition by default. Then, in the prepareOutTransition, we add a check for ScreenStackHeaderConfig and apply the transition to the children of its Toolbar making them stay on the screen for during the transition of going back. Fixes #817.

Changes

Added method to get the Toolbar of ScreenStackHeaderConfig.
Added check for getting the Toolbar in prepareOutTransition and firing the transition in its children.

Test code and steps to reproduce

Test817.tsx in TestsExample project.

Checklist

• Included code example that can be used to test this change
• Ensured that CI passes