System UI: Change corner resizability logic for region select.

changes in this CL:
- the knobs on the corners were removed as part of the UX change.
- the corners now have the ability to resize without depending on the knobs.
- the minimum size of the box is confirmed to be 1dp.

Screencast: http://shortn/_tfXqWcCjUe

Bug: 423965332,422565042,423964850
Test: Manual
Flag: com.android.systemui.desktop_screen_capture
Change-Id: I690cc48021c5353cf15c4abb9560dbd659cf36b5