Screen Capture: Create the region box in Compose.

This is a basic version, showing a rectangle on the screen to represent
the region selection. It is parameterized for width and height.

Screenshot: http://screen/7XpzXGfbD3Kmed4

Bug: 417533124
Test: Manual
Flag: com.android.systemui.desktop_screen_capture
Change-Id: Ibbe9d92789d59ab1b79230b864e66f3a7af5d02f