System UI: Add resizing capabilities to the region box for screen capture

Adding onDrag event to handle corner resize. I will address making the
opposite corners static so that dragging each corner would only resize
itself and its adjacent corners in a follow-up CL.

screencast: http://shortn/_YNAH5SrndB
The red background is for visibility only and is not included in this
CL.

Bug: 417533606
Test: Manual
Flag: com.android.systemui.desktop_screen_capture
Change-Id: Ie96f19308630a5ca25d152adb752d8ad32df280c