screen capture: update cursor for different cases during region capture

Change the cursor dynamically during partial screen capture:
   - Crosshair: When ready to draw or drawing a new selection region.
   - Move Icon: When hovering over the selected area for moving.
   - Resize Arrows: When hovering over the edges or corners of the
    selection box, change to diagonal, vertical, horizontal arrows.
   - Hand Icon: When hovering over the capture button.

The implementation is centralized within the RegionBox composable.
A pointerInput modifier continuously tracks the pointer's
position and updates a central state object (RegionBoxState) with
the current hover context (e.g., isHoveringButton, hoveredZone).

Bug: 423967244, 423967637
Test: Manual
Flag: com.android.systemui.large_screen_screencapture
Change-Id: Ic129e09ecb53d6d6908f27a01ed40e3d503af54b