System UI: Add knob to corners of screen capture region select component

This changes the Region Box composable to display 4 knobs in each of its
corner.

screenshot: http://screenshot/64AhTMPfihpymAo
the screenshot includes a red background color to make the box and knobs more visible.

Bug: 417533606
Test: Manual
Flag: com.android.systemui.desktop_screen_capture
Change-Id: Id26d44098aa36974fb0cdf9e5264960c5876bcc3