Adjust edit mode selection behavior.

This changelist updates the edit mode selection behavior with the
following changes:
- Widgets can be unselected by tapping the widget again. Previously,
clearing selection required tapping a blank spot.
- Remove/Add Widgets buttons are mutually exclusive and are never shown
at the same time.

Fixes: 383160667
Test: manual - verified new touch and layout changes.
Flag: com.android.systemui.hub_edit_mode_touch_adjustments
Change-Id: Idab26de851f9110f14196866ad073ad08519f204