screen capture: fix the dimension pill's position during draggin

Previously, the pill was always placed at a fixed vertical distance
below the selection box. This CL improved this logic by checking if
this default placement would cause the pill to be rendered partially or
completely off the bottom of the screen, and if so, it attempts to
place it always within the bounds of the screen.

The goal is to ensure the pill displaying the capture dimensions
remains visible on screen, even when the user draws a selection region
near the edges of the display.

Bug: 437975890
Test: Manual
Flag: com.android.systemui.large_screen_screencapture

Change-Id: If8543e8677c77860cede57685e95ce57667e6e79