System UI: Add knob to screen capture region select component

Adding a new white knob that will be placed on the corners of the region
box for the user to drag to resize the box.

Screenshot: http://screenshot/4DjZb2fSF7FBWqc

Bug: 417533606
Test: Manual
Flag: com.android.systemui.desktop_screen_capture
Change-Id: I7b7ce5b644e9141286fd61c2525ac3886c45c55d