System UI: allow users to draw the rectangle in region select

Users are now able to initiate and draw rectangles with their desired width and height.

Also, the callback now returns a geometry.rect object that is needed per the screenshot API.

The code is also refactored so all the drag handling is encapsulated in the `RegionBoxState` class.

Screen Recording: http://shortn/_TjdLuugMaY

Bug: 423964584
Test: Manual
Flag: com.android.systemui.desktop_screen_capture
Change-Id: I3549383559676a6ead8271f0976ba427dd642722