Desktop screenshots: Set the minimum region box size to 48dp

After dragging the region box, check if the height/width < 48dp. Resize
the box if the region box is too small. Also cap the size when resizing
the region box.

Bug: 435700935
Test: Manually on DUT
Flag: com.android.systemui.large_screen_screencapture
Change-Id: I6786a87f21e51d3353105ac64c0867ebcf87b38f