ComboBox drop down opens in PerMonitor awareness when hosted in System aware UI

[grubioe]

Copied from VSO ID:803149

Application Name: Mathcad Prime

OS: Windows 10 RS4

CPU: X64

.NET Build Number: 4.8.03707

Min Repro Project:
See VSO ID

Repro Steps:

Have .NET 4.7.2+, and 100% primary, 200% secondary
Launch repro proj exe on primary
Move it to secondary
Click “Launch Dialog”
Click ComboBox in dialog (Or tab to it and hit Alt+DownArrow if using keyboard)

Expected:

Popup is aligned correctly, and system scaled

Actual:

Popup is misaligned and PMA scaled

[vatsan-madhavan]

Moving to P9 since this will be a large/complex PR.