Tabbing through PMUI in the reverse should mirror forward direction

[donnie-msft]

Once you Shift+Tab to the TabControl, you can no longer Shift+Tab to return to the previously focused control.
Tab allows you to cycle through controls indefinitely.
/cc: @dominoFire @rrelyea

Bug: https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1241745

Environment Details:​
Microsoft Visual Studio Enterprise 2019 Preview
Version 16.8 Preview 6.0
OS Details:
Windows 10 Enterprise
Version : 20H2
Build: 19042.572

Repro Steps:​

1. Open Visual Studio Enterprise 2019 Preview (Version 16.8.0 Preview 6.0)
2. Create a new "Grpc" project
3. Tab till “Solution Explorer” and open "Context Menu".
4. Navigate to "Manage Nuget Packages for Solution" and press enter on it.
5. Navigate to "Update" tab using arrow key.
6. Search for "gRPC" packages.
7. Tab to all controls and navigate to "Update" tab from "Dependencies" tree view.
8. Navigate to "Dependencies" from "Update" tab using "Shift + Tab".
9. Verify whether user is able to navigate or not.

Actual Result:​

Unable to navigate to "Dependencies" tree view item from "Update" tab while shift tabbing.
Focus is staying on "Update" tab only.

Expected Result:​

User should be able to navigate to "Dependencies" tree view item from "Update" tab while shift tabbing.

User Impact:​

Keyboard users find difficulty in navigating to all controls if few controls are not navigable using "Shift + Tab"

MAS Reference:
MAS 2.4.3 – Focus Order