Desktop screenshots: Create view model method for partial screenshots

Creates the takePartialScreenshot() method in the view model which
handles the user's request to take a partial screenshot. The method
calls the ScreenshotInteractor which is ultimately responsible for
capturing the image.

Bug: 427544345
Test: atest PreCaptureViewModelTest
Flag: com.android.systemui.desktop_screen_capture
Change-Id: Iad170beebcd2e999664bd0998678f948407fc3c6