Desktop screenshots: Add a default region box

For partial screenshot, we add a default region box with the width and
height to be 50% of screen size, and place it at the screen center.

Screenshot: http://go/scrcast/NTM3Njg5NzE4MDQ5OTk2OHw2ODRkZDRiZC1hNw

Bug: 435494421
Test: Manually
Test: atest PreCaptureViewModelTest
Flag: com.android.systemui.large_screen_screencapture
Change-Id: I4ab64b97741e6cc702a26b67337041c896fa23e9